I have hitherto sometimes spoken as if the variations--so common and multiform in organic beings under domestication6, and in a lesser7 degree in those in a state of nature--had been due to chance. This, of course, is a wholly incorrect expression, but it serves to acknowledge plainly our ignorance of the cause of each particular variation. Some authors believe it to be as much the function of the reproductive system to produce individual differences, or very slight deviations9 of structure, as to make the child like its parents. But the much greater variability, as well as the greater frequency of monstrosities, under domestication or cultivation10, than under nature, leads me to believe that deviations of structure are in some way due to the nature of the conditions of life, to which the parents and their more remote ancestors have been exposed during several generations. I have remarked in the first chapter--but a long catalogue of facts which cannot be here given would be necessary to show the truth of the remark--that the reproductive system is eminently11 susceptible12 to changes in the conditions of life; and to this system being functionally13 disturbed in the parents, I chiefly attribute the varying or plastic condition of the offspring. The male and female sexual elements seem to be affected14 before that union takes place which is to form a new being. In the case of "sporting" plants, the bud, which in its earliest condition does not apparently15 differ essentially16 from an ovule, is alone affected. But why, because the reproductive system is disturbed, this or that part should vary more or less, we are profoundly ignorant. Nevertheless, we can here and there dimly catch a faint ray of light, and we may feel sure that there must be some cause for each deviation8 of structure, however slight.
How much direct effect difference of climate, food, etc., produces on any being is extremely doubtful. My impression is, that the effect is extremely small in the case of animals, but perhaps rather more in that of plants. We may, at least, safely conclude that such influences cannot have produced the many striking and complex co-adaptations of structure between one organic being and another, which we see everywhere throughout nature. Some little influence may be attributed to climate, food, etc.: thus, E. Forbes speaks confidently that shells at their southern limit, and when living in shallow water, are more brightly coloured than those of the same species further north or from greater depths. Gould believes that birds of the same species are more brightly coloured under a clear atmosphere, than when living on islands or near the coast. So with insects, Wollaston is convinced that residence near the sea affects their colours. Moquin-Tandon gives a list of plants which when growing near the sea-shore have their leaves in some degree fleshy, though not elsewhere fleshy. Several other such cases could be given.
The fact of varieties of one species, when they range into the zone of habitation of other species, often acquiring in a very slight degree some of the characters of such species, accords with our view that species of all kinds are only well-marked and permanent varieties. Thus the species of shells which are confined to tropical and shallow seas are generally brighter-coloured than those confined to cold and deeper seas. The birds which are confined to continents are, according to Mr. Gould, brighter-coloured than those of islands. The insect-species confined to sea-coasts, as every collector knows, are often brassy or lurid18. Plants which live exclusively on the sea-side are very apt to have fleshy leaves. He who believes in the creation of each species, will have to say that this shell, for instance, was created with bright colours for a warm sea; but that this other shell became bright-coloured by variation when it ranged into warmer or shallower waters.
When a variation is of the slightest use to a being, we cannot tell how much of it to attribute to the accumulative action of natural selection, and how much to the conditions of life. Thus, it is well known to furriers that animals of the same species have thicker and better fur the more severe the climate is under which they have lived; but who can tell how much of this difference may be due to the warmest-clad individuals having been favoured and preserved during many generations, and how much to the direct action of the severe climate? for it would appear that climate has some direct action on the hair of our domestic quadrupeds.
Instances could be given of the same variety being produced under conditions of life as different as can well be conceived; and, on the other hand, of different varieties being produced from the same species under the same conditions. Such facts show how indirectly19 the conditions of life must act. Again, innumerable instances are known to every naturalist20 of species keeping true, or not varying at all, although living under the most opposite climates. Such considerations as these incline me to lay very little weight on the direct action of the conditions of life. Indirectly, as already remarked, they seem to play an important part in affecting the reproductive system, and in thus inducing variability; and natural selection will then accumulate all profitable variations, however slight, until they become plainly developed and appreciable21 by us.
EFFECTS OF USE AND DISUSE.
From the facts alluded22 to in the first chapter, I think there can be little doubt that use in our domestic animals strengthens and enlarges certain parts, and disuse diminishes them; and that such modifications23 are inherited. Under free nature, we can have no standard of comparison, by which to judge of the effects of long-continued use or disuse, for we know not the parent-forms; but many animals have structures which can be explained by the effects of disuse. As Professor Owen has remarked, there is no greater anomaly in nature than a bird that cannot fly; yet there are several in this state. The logger-headed duck of South America can only flap along the surface of the water, and has its wings in nearly the same condition as the domestic Aylesbury duck. As the larger ground-feeding birds seldom take flight except to escape danger, I believe that the nearly wingless condition of several birds, which now inhabit or have lately inhabited several oceanic islands, tenanted by no beast of prey25, has been caused by disuse. The ostrich26 indeed inhabits continents and is exposed to danger from which it cannot escape by flight, but by kicking it can defend itself from enemies, as well as any of the smaller quadrupeds. We may imagine that the early progenitor27 of the ostrich had habits like those of a bustard, and that as natural selection increased in successive generations the size and weight of its body, its legs were used more, and its wings less, until they became incapable28 of flight.
Kirby has remarked (and I have observed the same fact) that the anterior29 tarsi, or feet, of many male dung-feeding beetles30 are very often broken off; he examined seventeen specimens33 in his own collection, and not one had even a relic34 left. In the Onites apelles the tarsi are so habitually35 lost, that the insect has been described as not having them. In some other genera they are present, but in a rudimentary condition. In the Ateuchus or sacred beetle31 of the Egyptians, they are totally deficient36. There is not sufficient evidence to induce us to believe that mutilations are ever inherited; and I should prefer explaining the entire absence of the anterior tarsi in Ateuchus, and their rudimentary condition in some other genera, by the long-continued effects of disuse in their progenitors37; for as the tarsi are almost always lost in many dung-feeding beetles, they must be lost early in life, and therefore cannot be much used by these insects.
In some cases we might easily put down to disuse modifications of structure which are wholly, or mainly, due to natural selection. Mr. Wollaston has discovered the remarkable38 fact that 200 beetles, out of the 550 species inhabiting Madeira, are so far deficient in wings that they cannot fly; and that of the twenty-nine endemic genera, no less than twenty-three genera have all their species in this condition! Several facts, namely, that beetles in many parts of the world are very frequently blown to sea and perish; that the beetles in Madeira, as observed by Mr. Wollaston, lie much concealed39, until the wind lulls40 and the sun shines; that the proportion of wingless beetles is larger on the exposed Dezertas than in Madeira itself; and especially the extraordinary fact, so strongly insisted on by Mr. Wollaston, of the almost entire absence of certain large groups of beetles, elsewhere excessively numerous, and which groups have habits of life almost necessitating42 frequent flight;--these several considerations have made me believe that the wingless condition of so many Madeira beetles is mainly due to the action of natural selection, but combined probably with disuse. For during thousands of successive generations each individual beetle which flew least, either from its wings having been ever so little less perfectly43 developed or from indolent habit, will have had the best chance of surviving from not being blown out to sea; and, on the other hand, those beetles which most readily took to flight will oftenest have been blown to sea and thus have been destroyed.
The insects in Madeira which are not ground-feeders, and which, as the flower-feeding coleoptera and lepidoptera, must habitually use their wings to gain their subsistence, have, as Mr. Wollaston suspects, their wings not at all reduced, but even enlarged. This is quite compatible with the action of natural selection. For when a new insect first arrived on the island, the tendency of natural selection to enlarge or to reduce the wings, would depend on whether a greater number of individuals were saved by successfully battling with the winds, or by giving up the attempt and rarely or never flying. As with mariners45 shipwrecked near a coast, it would have been better for the good swimmers if they had been able to swim still further, whereas it would have been better for the bad swimmers if they had not been able to swim at all and had stuck to the wreck46.
The eyes of moles47 and of some burrowing49 rodents50 are rudimentary in size, and in some cases are quite covered up by skin and fur. This state of the eyes is probably due to gradual reduction from disuse, but aided perhaps by natural selection. In South America, a burrowing rodent51, the tuco-tuco, or Ctenomys, is even more subterranean52 in its habits than the mole48; and I was assured by a Spaniard, who had often caught them, that they were frequently blind; one which I kept alive was certainly in this condition, the cause, as appeared on dissection53, having been inflammation of the nictitating membrane54. As frequent inflammation of the eyes must be injurious to any animal, and as eyes are certainly not indispensable to animals with subterranean habits, a reduction in their size with the adhesion of the eyelids55 and growth of fur over them, might in such case be an advantage; and if so, natural selection would constantly aid the effects of disuse.
It is well known that several animals, belonging to the most different classes, which inhabit the caves of Styria and of Kentucky, are blind. In some of the crabs57 the foot-stalk for the eye remains58, though the eye is gone; the stand for the telescope is there, though the telescope with its glasses has been lost. As it is difficult to imagine that eyes, though useless, could be in any way injurious to animals living in darkness, I attribute their loss wholly to disuse. In one of the blind animals, namely, the cave-rat, the eyes are of immense size; and Professor Silliman thought that it regained59, after living some days in the light, some slight power of vision. In the same manner as in Madeira the wings of some of the insects have been enlarged, and the wings of others have been reduced by natural selection aided by use and disuse, so in the case of the cave-rat natural selection seems to have struggled with the loss of light and to have increased the size of the eyes; whereas with all the other inhabitants of the caves, disuse by itself seems to have done its work.
It is difficult to imagine conditions of life more similar than deep limestone60 caverns61 under a nearly similar climate; so that on the common view of the blind animals having been separately created for the American and European caverns, close similarity in their organisation62 and affinities63 might have been expected; but, as Schiodte and others have remarked, this is not the case, and the cave-insects of the two continents are not more closely allied64 than might have been anticipated from the general resemblance of the other inhabitants of North America and Europe. On my view we must suppose that American animals, having ordinary powers of vision, slowly migrated by successive generations from the outer world into the deeper and deeper recesses65 of the Kentucky caves, as did European animals into the caves of Europe. We have some evidence of this gradation of habit; for, as Schiodte remarks, "animals not far remote from ordinary forms, prepare the transition from light to darkness. Next follow those that are constructed for twilight66; and, last of all, those destined67 for total darkness." By the time that an animal had reached, after numberless generations, the deepest recesses, disuse will on this view have more or less perfectly obliterated68 its eyes, and natural selection will often have effected other changes, such as an increase in the length of the antennae69 or palpi, as a compensation for blindness. Notwithstanding such modifications, we might expect still to see in the cave-animals of America, affinities to the other inhabitants of that continent, and in those of Europe, to the inhabitants of the European continent. And this is the case with some of the American cave-animals, as I hear from Professor Dana; and some of the European cave-insects are very closely allied to those of the surrounding country. It would be most difficult to give any rational explanation of the affinities of the blind cave-animals to the other inhabitants of the two continents on the ordinary view of their independent creation. That several of the inhabitants of the caves of the Old and New Worlds should be closely related, we might expect from the well-known relationship of most of their other productions. Far from feeling any surprise that some of the cave-animals should be very anomalous70, as Agassiz has remarked in regard to the blind fish, the Amblyopsis, and as is the case with the blind Proteus with reference to the reptiles71 of Europe, I am only surprised that more wrecks72 of ancient life have not been preserved, owing to the less severe competition to which the inhabitants of these dark abodes73 will probably have been exposed.
ACCLIMATISATION.
Habit is hereditary74 with plants, as in the period of flowering, in the amount of rain requisite75 for seeds to germinate76, in the time of sleep, etc., and this leads me to say a few words on acclimatisation. As it is extremely common for species of the same genus to inhabit very hot and very cold countries, and as I believe that all the species of the same genus have descended77 from a single parent, if this view be correct, acclimatisation must be readily effected during long-continued descent. It is notorious that each species is adapted to the climate of its own home: species from an arctic or even from a temperate78 region cannot endure a tropical climate, or conversely. So again, many succulent plants cannot endure a damp climate. But the degree of adaptation of species to the climates under which they live is often overrated. We may infer this from our frequent inability to predict whether or not an imported plant will endure our climate, and from the number of plants and animals brought from warmer countries which here enjoy good health. We have reason to believe that species in a state of nature are limited in their ranges by the competition of other organic beings quite as much as, or more than, by adaptation to particular climates. But whether or not the adaptation be generally very close, we have evidence, in the case of some few plants, of their becoming, to a certain extent, naturally habituated to different temperatures, or becoming acclimatised: thus the pines and rhododendrons, raised from seed collected by Dr. Hooker from trees growing at different heights on the Himalaya, were found in this country to possess different constitutional powers of resisting cold. Mr. Thwaites informs me that he has observed similar facts in Ceylon, and analogous observations have been made by Mr. H. C. Watson on European species of plants brought from the Azores to England. In regard to animals, several authentic79 cases could be given of species within historical times having largely extended their range from warmer to cooler latitudes80, and conversely; but we do not positively81 know that these animals were strictly82 adapted to their native climate, but in all ordinary cases we assume such to be the case; nor do we know that they have subsequently become acclimatised to their new homes.
As I believe that our domestic animals were originally chosen by uncivilised man because they were useful and bred readily under confinement83, and not because they were subsequently found capable of far-extended transportation, I think the common and extraordinary capacity in our domestic animals of not only withstanding the most different climates but of being perfectly fertile (a far severer test) under them, may be used as an argument that a large proportion of other animals, now in a state of nature, could easily be brought to bear widely different climates. We must not, however, push the foregoing argument too far, on account of the probable origin of some of our domestic animals from several wild stocks: the blood, for instance, of a tropical and arctic wolf or wild dog may perhaps be mingled84 in our domestic breeds. The rat and mouse cannot be considered as domestic animals, but they have been transported by man to many parts of the world, and now have a far wider range than any other rodent, living free under the cold climate of Faroe in the north and of the Falklands in the south, and on many islands in the torrid zones. Hence I am inclined to look at adaptation to any special climate as a quality readily grafted85 on an innate86 wide flexibility87 of constitution, which is common to most animals. On this view, the capacity of enduring the most different climates by man himself and by his domestic animals, and such facts as that former species of the elephant and rhinoceros88 were capable of enduring a glacial climate, whereas the living species are now all tropical or sub-tropical in their habits, ought not to be looked at as anomalies, but merely as examples of a very common flexibility of constitution, brought, under peculiar90 circumstances, into play.
How much of the acclimatisation of species to any peculiar climate is due to mere89 habit, and how much to the natural selection of varieties having different innate constitutions, and how much to both means combined, is a very obscure question. That habit or custom has some influence I must believe, both from analogy, and from the incessant91 advice given in agricultural works, even in the ancient Encyclopaedias92 of China, to be very cautious in transposing animals from one district to another; for it is not likely that man should have succeeded in selecting so many breeds and sub-breeds with constitutions specially41 fitted for their own districts: the result must, I think, be due to habit. On the other hand, I can see no reason to doubt that natural selection will continually tend to preserve those individuals which are born with constitutions best adapted to their native countries. In treatises94 on many kinds of cultivated plants, certain varieties are said to withstand certain climates better than others: this is very strikingly shown in works on fruit trees published in the United States, in which certain varieties are habitually recommended for the northern, and others for the southern States; and as most of these varieties are of recent origin, they cannot owe their constitutional differences to habit. The case of the Jerusalem artichoke, which is never propagated by seed, and of which consequently new varieties have not been produced, has even been advanced--for it is now as tender as ever it was--as proving that acclimatisation cannot be effected! The case, also, of the kidney-bean has been often cited for a similar purpose, and with much greater weight; but until some one will sow, during a score of generations, his kidney-beans so early that a very large proportion are destroyed by frost, and then collect seed from the few survivors95, with care to prevent accidental crosses, and then again get seed from these seedlings96, with the same precautions, the experiment cannot be said to have been even tried. Nor let it be supposed that no differences in the constitution of seedling97 kidney-beans ever appear, for an account has been published how much more hardy98 some seedlings appeared to be than others.
On the whole, I think we may conclude that habit, use, and disuse, have, in some cases, played a considerable part in the modification24 of the constitution, and of the structure of various organs; but that the effects of use and disuse have often been largely combined with, and sometimes overmastered by, the natural selection of innate differences.
CORRELATION OF GROWTH.
I mean by this expression that the whole organisation is so tied together during its growth and development, that when slight variations in any one part occur, and are accumulated through natural selection, other parts become modified. This is a very important subject, most imperfectly understood. The most obvious case is, that modifications accumulated solely99 for the good of the young or larva, will, it may safely be concluded, affect the structure of the adult; in the same manner as any malconformation affecting the early embryo100, seriously affects the whole organisation of the adult. The several parts of the body which are homologous, and which, at an early embryonic101 period, are alike, seem liable to vary in an allied manner: we see this in the right and left sides of the body varying in the same manner; in the front and hind102 legs, and even in the jaws103 and limbs, varying together, for the lower jaw104 is believed to be homologous with the limbs. These tendencies, I do not doubt, may be mastered more or less completely by natural selection: thus a family of stags once existed with an antler only on one side; and if this had been of any great use to the breed it might probably have been rendered permanent by natural selection.
Homologous parts, as has been remarked by some authors, tend to cohere105; this is often seen in monstrous106 plants; and nothing is more common than the union of homologous parts in normal structures, as the union of the petals107 of the corolla into a tube. Hard parts seem to affect the form of adjoining soft parts; it is believed by some authors that the diversity in the shape of the pelvis in birds causes the remarkable diversity in the shape of their kidneys. Others believe that the shape of the pelvis in the human mother influences by pressure the shape of the head of the child. In snakes, according to Schlegel, the shape of the body and the manner of swallowing determine the position of several of the most important viscera.
The nature of the bond of correlation is very frequently quite obscure. M. Is. Geoffroy St. Hilaire has forcibly remarked, that certain malconformations very frequently, and that others rarely coexist, without our being able to assign any reason. What can be more singular than the relation between blue eyes and deafness in cats, and the tortoise-shell colour with the female sex; the feathered feet and skin between the outer toes in pigeons, and the presence of more or less down on the young birds when first hatched, with the future colour of their plumage; or, again, the relation between the hair and teeth in the naked Turkish dog, though here probably homology comes into play? With respect to this latter case of correlation, I think it can hardly be accidental, that if we pick out the two orders of mammalia which are most abnormal in their dermal108 coverings, viz. Cetacea (whales) and Edentata (armadilloes, scaly109 ant-eaters, etc.), that these are likewise the most abnormal in their teeth.
I know of no case better adapted to show the importance of the laws of correlation in modifying important structures, independently of utility and, therefore, of natural selection, than that of the difference between the outer and inner flowers in some Compositous and Umbelliferous plants. Every one knows the difference in the ray and central florets of, for instance, the daisy, and this difference is often accompanied with the abortion110 of parts of the flower. But, in some Compositous plants, the seeds also differ in shape and sculpture; and even the ovary itself, with its accessory parts, differs, as has been described by Cassini. These differences have been attributed by some authors to pressure, and the shape of the seeds in the ray-florets in some Compositae countenances111 this idea; but, in the case of the corolla of the Umbelliferae, it is by no means, as Dr. Hooker informs me, in species with the densest112 heads that the inner and outer flowers most frequently differ. It might have been thought that the development of the ray-petals by drawing nourishment113 from certain other parts of the flower had caused their abortion; but in some Compositae there is a difference in the seeds of the outer and inner florets without any difference in the corolla. Possibly, these several differences may be connected with some difference in the flow of nutriment towards the central and external flowers: we know, at least, that in irregular flowers, those nearest to the axis114 are oftenest subject to peloria, and become regular. I may add, as an instance of this, and of a striking case of correlation, that I have recently observed in some garden pelargoniums, that the central flower of the truss often loses the patches of darker colour in the two upper petals; and that when this occurs, the adherent115 nectary is quite aborted116; when the colour is absent from only one of the two upper petals, the nectary is only much shortened.
With respect to the difference in the corolla of the central and exterior117 flowers of a head or umbel, I do not feel at all sure that C. C. Sprengel's idea that the ray-florets serve to attract insects, whose agency is highly advantageous118 in the fertilisation of plants of these two orders, is so far-fetched, as it may at first appear: and if it be advantageous, natural selection may have come into play. But in regard to the differences both in the internal and external structure of the seeds, which are not always correlated with any differences in the flowers, it seems impossible that they can be in any way advantageous to the plant: yet in the Umbelliferae these differences are of such apparent importance--the seeds being in some cases, according to Tausch, orthospermous in the exterior flowers and coelospermous in the central flowers,--that the elder De Candolle founded his main divisions of the order on analogous differences. Hence we see that modifications of structure, viewed by systematists as of high value, may be wholly due to unknown laws of correlated growth, and without being, as far as we can see, of the slightest service to the species.
We may often falsely attribute to correlation of growth, structures which are common to whole groups of species, and which in truth are simply due to inheritance; for an ancient progenitor may have acquired through natural selection some one modification in structure, and, after thousands of generations, some other and independent modification; and these two modifications, having been transmitted to a whole group of descendants with diverse habits, would naturally be thought to be correlated in some necessary manner. So, again, I do not doubt that some apparent correlations, occurring throughout whole orders, are entirely119 due to the manner alone in which natural selection can act. For instance, Alph. De Candolle has remarked that winged seeds are never found in fruits which do not open: I should explain the rule by the fact that seeds could not gradually become winged through natural selection, except in fruits which opened; so that the individual plants producing seeds which were a little better fitted to be wafted120 further, might get an advantage over those producing seed less fitted for dispersal; and this process could not possibly go on in fruit which did not open.
The elder Geoffroy and Goethe propounded121, at about the same period, their law of compensation or balancement of growth; or, as Goethe expressed it, "in order to spend on one side, nature is forced to economise on the other side." I think this holds true to a certain extent with our domestic productions: if nourishment flows to one part or organ in excess, it rarely flows, at least in excess, to another part; thus it is difficult to get a cow to give much milk and to fatten122 readily. The same varieties of the cabbage do not yield abundant and nutritious123 foliage124 and a copious125 supply of oil-bearing seeds. When the seeds in our fruits become atrophied126, the fruit itself gains largely in size and quality. In our poultry127, a large tuft of feathers on the head is generally accompanied by a diminished comb, and a large beard by diminished wattles. With species in a state of nature it can hardly be maintained that the law is of universal application; but many good observers, more especially botanists128, believe in its truth. I will not, however, here give any instances, for I see hardly any way of distinguishing between the effects, on the one hand, of a part being largely developed through natural selection and another and adjoining part being reduced by this same process or by disuse, and, on the other hand, the actual withdrawal129 of nutriment from one part owing to the excess of growth in another and adjoining part.
I suspect, also, that some of the cases of compensation which have been advanced, and likewise some other facts, may be merged130 under a more general principle, namely, that natural selection is continually trying to economise in every part of the organisation. If under changed conditions of life a structure before useful becomes less useful, any diminution131, however slight, in its development, will be seized on by natural selection, for it will profit the individual not to have its nutriment wasted in building up an useless structure. I can thus only understand a fact with which I was much struck when examining cirripedes, and of which many other instances could be given: namely, that when a cirripede is parasitic132 within another and is thus protected, it loses more or less completely its own shell or carapace133. This is the case with the male Ibla, and in a truly extraordinary manner with the Proteolepas: for the carapace in all other cirripedes consists of the three highly-important anterior segments of the head enormously developed, and furnished with great nerves and muscles; but in the parasitic and protected Proteolepas, the whole anterior part of the head is reduced to the merest rudiment3 attached to the bases of the prehensile134 antennae. Now the saving of a large and complex structure, when rendered superfluous135 by the parasitic habits of the Proteolepas, though effected by slow steps, would be a decided136 advantage to each successive individual of the species; for in the struggle for life to which every animal is exposed, each individual Proteolepas would have a better chance of supporting itself, by less nutriment being wasted in developing a structure now become useless.
Thus, as I believe, natural selection will always succeed in the long run in reducing and saving every part of the organisation, as soon as it is rendered superfluous, without by any means causing some other part to be largely developed in a corresponding degree. And, conversely, that natural selection may perfectly well succeed in largely developing any organ, without requiring as a necessary compensation the reduction of some adjoining part.
It seems to be a rule, as remarked by Is. Geoffroy St. Hilaire, both in varieties and in species, that when any part or organ is repeated many times in the structure of the same individual (as the vertebrae in snakes, and the stamens in polyandrous flowers) the number is variable; whereas the number of the same part or organ, when it occurs in lesser numbers, is constant. The same author and some botanists have further remarked that multiple parts are also very liable to variation in structure. Inasmuch as this "vegetative repetition," to use Professor Owen's expression, seems to be a sign of low organisation; the foregoing remark seems connected with the very general opinion of naturalists137, that beings low in the scale of nature are more variable than those which are higher. I presume that lowness in this case means that the several parts of the organisation have been but little specialised for particular functions; and as long as the same part has to perform diversified138 work, we can perhaps see why it should remain variable, that is, why natural selection should have preserved or rejected each little deviation of form less carefully than when the part has to serve for one special purpose alone. In the same way that a knife which has to cut all sorts of things may be of almost any shape; whilst a tool for some particular object had better be of some particular shape. Natural selection, it should never be forgotten, can act on each part of each being, solely through and for its advantage.
Rudimentary parts, it has been stated by some authors, and I believe with truth, are apt to be highly variable. We shall have to recur139 to the general subject of rudimentary and aborted organs; and I will here only add that their variability seems to be owing to their uselessness, and therefore to natural selection having no power to check deviations in their structure. Thus rudimentary parts are left to the free play of the various laws of growth, to the effects of long-continued disuse, and to the tendency to reversion.
A PART DEVELOPED IN ANY SPECIES IN AN EXTRAORDINARY DEGREE OR MANNER, IN COMPARISON WITH THE SAME PART IN ALLIED SPECIES, TENDS TO BE HIGHLY VARIABLE.
Several years ago I was much struck with a remark, nearly to the above effect, published by Mr. Waterhouse. I infer also from an observation made by Professor Owen, with respect to the length of the arms of the ourang-outang, that he has come to a nearly similar conclusion. It is hopeless to attempt to convince any one of the truth of this proposition without giving the long array of facts which I have collected, and which cannot possibly be here introduced. I can only state my conviction that it is a rule of high generality. I am aware of several causes of error, but I hope that I have made due allowance for them. It should be understood that the rule by no means applies to any part, however unusually developed, unless it be unusually developed in comparison with the same part in closely allied species. Thus, the bat's wing is a most abnormal structure in the class mammalia; but the rule would not here apply, because there is a whole group of bats having wings; it would apply only if some one species of bat had its wings developed in some remarkable manner in comparison with the other species of the same genus. The rule applies very strongly in the case of secondary sexual characters, when displayed in any unusual manner. The term, secondary sexual characters, used by Hunter, applies to characters which are attached to one sex, but are not directly connected with the act of reproduction. The rule applies to males and females; but as females more rarely offer remarkable secondary sexual characters, it applies more rarely to them. The rule being so plainly applicable in the case of secondary sexual characters, may be due to the great variability of these characters, whether or not displayed in any unusual manner--of which fact I think there can be little doubt. But that our rule is not confined to secondary sexual characters is clearly shown in the case of hermaphrodite cirripedes; and I may here add, that I particularly attended to Mr. Waterhouse's remark, whilst investigating this Order, and I am fully44 convinced that the rule almost invariably holds good with cirripedes. I shall, in my future work, give a list of the more remarkable cases; I will here only briefly140 give one, as it illustrates141 the rule in its largest application. The opercular valves of sessile cirripedes (rock barnacles) are, in every sense of the word, very important structures, and they differ extremely little even in different genera; but in the several species of one genus, Pyrgoma, these valves present a marvellous amount of diversification142: the homologous valves in the different species being sometimes wholly unlike in shape; and the amount of variation in the individuals of several of the species is so great, that it is no exaggeration to state that the varieties differ more from each other in the characters of these important valves than do other species of distinct genera.
As birds within the same country vary in a remarkably143 small degree, I have particularly attended to them, and the rule seems to me certainly to hold good in this class. I cannot make out that it applies to plants, and this would seriously have shaken my belief in its truth, had not the great variability in plants made it particularly difficult to compare their relative degrees of variability.
When we see any part or organ developed in a remarkable degree or manner in any species, the fair presumption144 is that it is of high importance to that species; nevertheless the part in this case is eminently liable to variation. Why should this be so? On the view that each species has been independently created, with all its parts as we now see them, I can see no explanation. But on the view that groups of species have descended from other species, and have been modified through natural selection, I think we can obtain some light. In our domestic animals, if any part, or the whole animal, be neglected and no selection be applied145, that part (for instance, the comb in the Dorking fowl) or the whole breed will cease to have a nearly uniform character. The breed will then be said to have degenerated146. In rudimentary organs, and in those which have been but little specialised for any particular purpose, and perhaps in polymorphic groups, we see a nearly parallel natural case; for in such cases natural selection either has not or cannot come into full play, and thus the organisation is left in a fluctuating condition. But what here more especially concerns us is, that in our domestic animals those points, which at the present time are undergoing rapid change by continued selection, are also eminently liable to variation. Look at the breeds of the pigeon; see what a prodigious147 amount of difference there is in the beak148 of the different tumblers, in the beak and wattle of the different carriers, in the carriage and tail of our fantails, etc., these being the points now mainly attended to by English fanciers. Even in the sub-breeds, as in the short-faced tumbler, it is notoriously difficult to breed them nearly to perfection, and frequently individuals are born which depart widely from the standard. There may be truly said to be a constant struggle going on between, on the one hand, the tendency to reversion to a less modified state, as well as an innate tendency to further variability of all kinds, and, on the other hand, the power of steady selection to keep the breed true. In the long run selection gains the day, and we do not expect to fail so far as to breed a bird as coarse as a common tumbler from a good short-faced strain. But as long as selection is rapidly going on, there may always be expected to be much variability in the structure undergoing modification. It further deserves notice that these variable characters, produced by man's selection, sometimes become attached, from causes quite unknown to us, more to one sex than to the other, generally to the male sex, as with the wattle of carriers and the enlarged crop of pouters.
Now let us turn to nature. When a part has been developed in an extraordinary manner in any one species, compared with the other species of the same genus, we may conclude that this part has undergone an extraordinary amount of modification, since the period when the species branched off from the common progenitor of the genus. This period will seldom be remote in any extreme degree, as species very rarely endure for more than one geological period. An extraordinary amount of modification implies an unusually large and long-continued amount of variability, which has continually been accumulated by natural selection for the benefit of the species. But as the variability of the extraordinarily149-developed part or organ has been so great and long-continued within a period not excessively remote, we might, as a general rule, expect still to find more variability in such parts than in other parts of the organisation, which have remained for a much longer period nearly constant. And this, I am convinced, is the case. That the struggle between natural selection on the one hand, and the tendency to reversion and variability on the other hand, will in the course of time cease; and that the most abnormally developed organs may be made constant, I can see no reason to doubt. Hence when an organ, however abnormal it may be, has been transmitted in approximately the same condition to many modified descendants, as in the case of the wing of the bat, it must have existed, according to my theory, for an immense period in nearly the same state; and thus it comes to be no more variable than any other structure. It is only in those cases in which the modification has been comparatively recent and extraordinarily great that we ought to find the GENERATIVE VARIABILITY, as it may be called, still present in a high degree. For in this case the variability will seldom as yet have been fixed150 by the continued selection of the individuals varying in the required manner and degree, and by the continued rejection151 of those tending to revert152 to a former and less modified condition.
The principle included in these remarks may be extended. It is notorious that specific characters are more variable than generic. To explain by a simple example what is meant. If some species in a large genus of plants had blue flowers and some had red, the colour would be only a specific character, and no one would be surprised at one of the blue species varying into red, or conversely; but if all the species had blue flowers, the colour would become a generic character, and its variation would be a more unusual circumstance. I have chosen this example because an explanation is not in this case applicable, which most naturalists would advance, namely, that specific characters are more variable than generic, because they are taken from parts of less physiological153 importance than those commonly used for classing genera. I believe this explanation is partly, yet only indirectly, true; I shall, however, have to return to this subject in our chapter on Classification. It would be almost superfluous to adduce evidence in support of the above statement, that specific characters are more variable than generic; but I have repeatedly noticed in works on natural history, that when an author has remarked with surprise that some IMPORTANT organ or part, which is generally very constant throughout large groups of species, has DIFFERED considerably154 in closely-allied species, that it has, also, been VARIABLE in the individuals of some of the species. And this fact shows that a character, which is generally of generic value, when it sinks in value and becomes only of specific value, often becomes variable, though its physiological importance may remain the same. Something of the same kind applies to monstrosities: at least Is. Geoffroy St. Hilaire seems to entertain no doubt, that the more an organ normally differs in the different species of the same group, the more subject it is to individual anomalies.
On the ordinary view of each species having been independently created, why should that part of the structure, which differs from the same part in other independently-created species of the same genus, be more variable than those parts which are closely alike in the several species? I do not see that any explanation can be given. But on the view of species being only strongly marked and fixed varieties, we might surely expect to find them still often continuing to vary in those parts of their structure which have varied155 within a moderately recent period, and which have thus come to differ. Or to state the case in another manner:--the points in which all the species of a genus resemble each other, and in which they differ from the species of some other genus, are called generic characters; and these characters in common I attribute to inheritance from a common progenitor, for it can rarely have happened that natural selection will have modified several species, fitted to more or less widely-different habits, in exactly the same manner: and as these so-called generic characters have been inherited from a remote period, since that period when the species first branched off from their common progenitor, and subsequently have not varied or come to differ in any degree, or only in a slight degree, it is not probable that they should vary at the present day. On the other hand, the points in which species differ from other species of the same genus, are called specific characters; and as these specific characters have varied and come to differ within the period of the branching off of the species from a common progenitor, it is probable that they should still often be in some degree variable,--at least more variable than those parts of the organisation which have for a very long period remained constant.
In connexion with the present subject, I will make only two other remarks. I think it will be admitted, without my entering on details, that secondary sexual characters are very variable; I think it also will be admitted that species of the same group differ from each other more widely in their secondary sexual characters, than in other parts of their organisation; compare, for instance, the amount of difference between the males of gallinaceous birds, in which secondary sexual characters are strongly displayed, with the amount of difference between their females; and the truth of this proposition will be granted. The cause of the original variability of secondary sexual characters is not manifest; but we can see why these characters should not have been rendered as constant and uniform as other parts of the organisation; for secondary sexual characters have been accumulated by sexual selection, which is less rigid156 in its action than ordinary selection, as it does not entail157 death, but only gives fewer offspring to the less favoured males. Whatever the cause may be of the variability of secondary sexual characters, as they are highly variable, sexual selection will have had a wide scope for action, and may thus readily have succeeded in giving to the species of the same group a greater amount of difference in their sexual characters, than in other parts of their structure.
It is a remarkable fact, that the secondary sexual differences between the two sexes of the same species are generally displayed in the very same parts of the organisation in which the different species of the same genus differ from each other. Of this fact I will give in illustration two instances, the first which happen to stand on my list; and as the differences in these cases are of a very unusual nature, the relation can hardly be accidental. The same number of joints158 in the tarsi is a character generally common to very large groups of beetles, but in the Engidae, as Westwood has remarked, the number varies greatly; and the number likewise differs in the two sexes of the same species: again in fossorial hymenoptera, the manner of neuration of the wings is a character of the highest importance, because common to large groups; but in certain genera the neuration differs in the different species, and likewise in the two sexes of the same species. This relation has a clear meaning on my view of the subject: I look at all the species of the same genus as having as certainly descended from the same progenitor, as have the two sexes of any one of the species. Consequently, whatever part of the structure of the common progenitor, or of its early descendants, became variable; variations of this part would it is highly probable, be taken advantage of by natural and sexual selection, in order to fit the several species to their several places in the economy of nature, and likewise to fit the two sexes of the same species to each other, or to fit the males and females to different habits of life, or the males to struggle with other males for the possession of the females.
Finally, then, I conclude that the greater variability of specific characters, or those which distinguish species from species, than of generic characters, or those which the species possess in common;--that the frequent extreme variability of any part which is developed in a species in an extraordinary manner in comparison with the same part in its congeners; and the not great degree of variability in a part, however extraordinarily it may be developed, if it be common to a whole group of species;--that the great variability of secondary sexual characters, and the great amount of difference in these same characters between closely allied species;--that secondary sexual and ordinary specific differences are generally displayed in the same parts of the organisation,--are all principles closely connected together. All being mainly due to the species of the same group having descended from a common progenitor, from whom they have inherited much in common,--to parts which have recently and largely varied being more likely still to go on varying than parts which have long been inherited and have not varied,--to natural selection having more or less completely, according to the lapse159 of time, overmastered the tendency to reversion and to further variability,--to sexual selection being less rigid than ordinary selection,--and to variations in the same parts having been accumulated by natural and sexual selection, and thus adapted for secondary sexual, and for ordinary specific purposes.
DISTINCT SPECIES PRESENT ANALOGOUS VARIATIONS; AND A VARIETY OF ONE SPECIES OFTEN ASSUMES SOME OF THE CHARACTERS OF AN ALLIED SPECIES, OR REVERTS160 TO SOME OF THE CHARACTERS OF AN EARLY PROGENITOR.
These propositions will be most readily understood by looking to our domestic races. The most distinct breeds of pigeons, in countries most widely apart, present sub-varieties with reversed feathers on the head and feathers on the feet,--characters not possessed161 by the aboriginal162 rock-pigeon; these then are analogous variations in two or more distinct races. The frequent presence of fourteen or even sixteen tail-feathers in the pouter, may be considered as a variation representing the normal structure of another race, the fantail. I presume that no one will doubt that all such analogous variations are due to the several races of the pigeon having inherited from a common parent the same constitution and tendency to variation, when acted on by similar unknown influences. In the vegetable kingdom we have a case of analogous variation, in the enlarged stems, or roots as commonly called, of the Swedish turnip163 and Ruta baga, plants which several botanists rank as varieties produced by cultivation from a common parent: if this be not so, the case will then be one of analogous variation in two so-called distinct species; and to these a third may be added, namely, the common turnip. According to the ordinary view of each species having been independently created, we should have to attribute this similarity in the enlarged stems of these three plants, not to the vera causa of community of descent, and a consequent tendency to vary in a like manner, but to three separate yet closely related acts of creation.
With pigeons, however, we have another case, namely, the occasional appearance in all the breeds, of slaty-blue birds with two black bars on the wings, a white rump, a bar at the end of the tail, with the outer feathers externally edged near their bases with white. As all these marks are characteristic of the parent rock-pigeon, I presume that no one will doubt that this is a case of reversion, and not of a new yet analogous variation appearing in the several breeds. We may I think confidently come to this conclusion, because, as we have seen, these coloured marks are eminently liable to appear in the crossed offspring of two distinct and differently coloured breeds; and in this case there is nothing in the external conditions of life to cause the reappearance of the slaty-blue, with the several marks, beyond the influence of the mere act of crossing on the laws of inheritance.
No doubt it is a very surprising fact that characters should reappear after having been lost for many, perhaps for hundreds of generations. But when a breed has been crossed only once by some other breed, the offspring occasionally show a tendency to revert in character to the foreign breed for many generations--some say, for a dozen or even a score of generations. After twelve generations, the proportion of blood, to use a common expression, of any one ancestor, is only 1 in 2048; and yet, as we see, it is generally believed that a tendency to reversion is retained by this very small proportion of foreign blood. In a breed which has not been crossed, but in which BOTH parents have lost some character which their progenitor possessed, the tendency, whether strong or weak, to reproduce the lost character might be, as was formerly164 remarked, for all that we can see to the contrary, transmitted for almost any number of generations. When a character which has been lost in a breed, reappears after a great number of generations, the most probable hypothesis is, not that the offspring suddenly takes after an ancestor some hundred generations distant, but that in each successive generation there has been a tendency to reproduce the character in question, which at last, under unknown favourable165 conditions, gains an ascendancy166. For instance, it is probable that in each generation of the barb-pigeon, which produces most rarely a blue and black-barred bird, there has been a tendency in each generation in the plumage to assume this colour. This view is hypothetical, but could be supported by some facts; and I can see no more abstract improbability in a tendency to produce any character being inherited for an endless number of generations, than in quite useless or rudimentary organs being, as we all know them to be, thus inherited. Indeed, we may sometimes observe a mere tendency to produce a rudiment inherited: for instance, in the common snapdragon (Antirrhinum) a rudiment of a fifth stamen so often appears, that this plant must have an inherited tendency to produce it.
As all the species of the same genus are supposed, on my theory, to have descended from a common parent, it might be expected that they would occasionally vary in an analogous manner; so that a variety of one species would resemble in some of its characters another species; this other species being on my view only a well-marked and permanent variety. But characters thus gained would probably be of an unimportant nature, for the presence of all important characters will be governed by natural selection, in accordance with the diverse habits of the species, and will not be left to the mutual167 action of the conditions of life and of a similar inherited constitution. It might further be expected that the species of the same genus would occasionally exhibit reversions to lost ancestral characters. As, however, we never know the exact character of the common ancestor of a group, we could not distinguish these two cases: if, for instance, we did not know that the rock-pigeon was not feather-footed or turn-crowned, we could not have told, whether these characters in our domestic breeds were reversions or only analogous variations; but we might have inferred that the blueness was a case of reversion, from the number of the markings, which are correlated with the blue tint168, and which it does not appear probable would all appear together from simple variation. More especially we might have inferred this, from the blue colour and marks so often appearing when distinct breeds of diverse colours are crossed. Hence, though under nature it must generally be left doubtful, what cases are reversions to an anciently existing character, and what are new but analogous variations, yet we ought, on my theory, sometimes to find the varying offspring of a species assuming characters (either from reversion or from analogous variation) which already occur in some other members of the same group. And this undoubtedly169 is the case in nature.
A considerable part of the difficulty in recognising a variable species in our systematic170 works, is due to its varieties mocking, as it were, some of the other species of the same genus. A considerable catalogue, also, could be given of forms intermediate between two other forms, which themselves must be doubtfully ranked as either varieties or species; and this shows, unless all these forms be considered as independently created species, that the one in varying has assumed some of the characters of the other, so as to produce the intermediate form. But the best evidence is afforded by parts or organs of an important and uniform nature occasionally varying so as to acquire, in some degree, the character of the same part or organ in an allied species. I have collected a long list of such cases; but here, as before, I lie under a great disadvantage in not being able to give them. I can only repeat that such cases certainly do occur, and seem to me very remarkable.
I will, however, give one curious and complex case, not indeed as affecting any important character, but from occurring in several species of the same genus, partly under domestication and partly under nature. It is a case apparently of reversion. The ass17 not rarely has very distinct transverse bars on its legs, like those on the legs of a zebra: it has been asserted that these are plainest in the foal, and from inquiries171 which I have made, I believe this to be true. It has also been asserted that the stripe on each shoulder is sometimes double. The shoulder stripe is certainly very variable in length and outline. A white ass, but NOT an albino, has been described without either spinal172 or shoulder-stripe; and these stripes are sometimes very obscure, or actually quite lost, in dark-coloured asses56. The koulan of Pallas is said to have been seen with a double shoulder-stripe. The hemionus has no shoulder-stripe; but traces of it, as stated by Mr. Blyth and others, occasionally appear: and I have been informed by Colonel Poole that the foals of this species are generally striped on the legs, and faintly on the shoulder. The quagga, though so plainly barred like a zebra over the body, is without bars on the legs; but Dr. Gray has figured one specimen32 with very distinct zebra-like bars on the hocks.
With respect to the horse, I have collected cases in England of the spinal stripe in horses of the most distinct breeds, and of ALL colours; transverse bars on the legs are not rare in duns, mouse-duns, and in one instance in a chestnut173: a faint shoulder-stripe may sometimes be seen in duns, and I have seen a trace in a bay horse. My son made a careful examination and sketch174 for me of a dun Belgian cart-horse with a double stripe on each shoulder and with leg-stripes; and a man, whom I can implicitly175 trust, has examined for me a small dun Welch pony176 with THREE short parallel stripes on each shoulder.
In the north-west part of India the Kattywar breed of horses is so generally striped, that, as I hear from Colonel Poole, who examined the breed for the Indian Government, a horse without stripes is not considered as purely-bred. The spine177 is always striped; the legs are generally barred; and the shoulder-stripe, which is sometimes double and sometimes treble, is common; the side of the face, moreover, is sometimes striped. The stripes are plainest in the foal; and sometimes quite disappear in old horses. Colonel Poole has seen both gray and bay Kattywar horses striped when first foaled. I have, also, reason to suspect, from information given me by Mr. W. W. Edwards, that with the English race-horse the spinal stripe is much commoner in the foal than in the full-grown animal. Without here entering on further details, I may state that I have collected cases of leg and shoulder stripes in horses of very different breeds, in various countries from Britain to Eastern China; and from Norway in the north to the Malay Archipelago in the south. In all parts of the world these stripes occur far oftenest in duns and mouse-duns; by the term dun a large range of colour is included, from one between brown and black to a close approach to cream-colour.
I am aware that Colonel Hamilton Smith, who has written on this subject, believes that the several breeds of the horse have descended from several aboriginal species--one of which, the dun, was striped; and that the above-described appearances are all due to ancient crosses with the dun stock. But I am not at all satisfied with this theory, and should be loth to apply it to breeds so distinct as the heavy Belgian cart-horse, Welch ponies178, cobs, the lanky179 Kattywar race, etc., inhabiting the most distant parts of the world.
Now let us turn to the effects of crossing the several species of the horse-genus. Rollin asserts, that the common mule180 from the ass and horse is particularly apt to have bars on its legs. I once saw a mule with its legs so much striped that any one at first would have thought that it must have been the product of a zebra; and Mr. W. C. Martin, in his excellent treatise93 on the horse, has given a figure of a similar mule. In four coloured drawings, which I have seen, of hybrids181 between the ass and zebra, the legs were much more plainly barred than the rest of the body; and in one of them there was a double shoulder-stripe. In Lord Moreton's famous hybrid182 from a chestnut mare183 and male quagga, the hybrid, and even the pure offspring subsequently produced from the mare by a black Arabian sire, were much more plainly barred across the legs than is even the pure quagga. Lastly, and this is another most remarkable case, a hybrid has been figured by Dr. Gray (and he informs me that he knows of a second case) from the ass and the hemionus; and this hybrid, though the ass seldom has stripes on its legs and the hemionus has none and has not even a shoulder-stripe, nevertheless had all four legs barred, and had three short shoulder-stripes, like those on the dun Welch pony, and even had some zebra-like stripes on the sides of its face. With respect to this last fact, I was so convinced that not even a stripe of colour appears from what would commonly be called an accident, that I was led solely from the occurrence of the face-stripes on this hybrid from the ass and hemionus, to ask Colonel Poole whether such face-stripes ever occur in the eminently striped Kattywar breed of horses, and was, as we have seen, answered in the affirmative.
What now are we to say to these several facts? We see several very distinct species of the horse-genus becoming, by simple variation, striped on the legs like a zebra, or striped on the shoulders like an ass. In the horse we see this tendency strong whenever a dun tint appears--a tint which approaches to that of the general colouring of the other species of the genus. The appearance of the stripes is not accompanied by any change of form or by any other new character. We see this tendency to become striped most strongly displayed in hybrids from between several of the most distinct species. Now observe the case of the several breeds of pigeons: they are descended from a pigeon (including two or three sub-species or geographical184 races) of a bluish colour, with certain bars and other marks; and when any breed assumes by simple variation a bluish tint, these bars and other marks invariably reappear; but without any other change of form or character. When the oldest and truest breeds of various colours are crossed, we see a strong tendency for the blue tint and bars and marks to reappear in the mongrels. I have stated that the most probable hypothesis to account for the reappearance of very ancient characters, is--that there is a TENDENCY in the young of each successive generation to produce the long-lost character, and that this tendency, from unknown causes, sometimes prevails. And we have just seen that in several species of the horse-genus the stripes are either plainer or appear more commonly in the young than in the old. Call the breeds of pigeons, some of which have bred true for centuries, species; and how exactly parallel is the case with that of the species of the horse-genus! For myself, I venture confidently to look back thousands on thousands of generations, and I see an animal striped like a zebra, but perhaps otherwise very differently constructed, the common parent of our domestic horse, whether or not it be descended from one or more wild stocks, of the ass, the hemionus, quagga, and zebra.
He who believes that each equine species was independently created, will, I presume, assert that each species has been created with a tendency to vary, both under nature and under domestication, in this particular manner, so as often to become striped like other species of the genus; and that each has been created with a strong tendency, when crossed with species inhabiting distant quarters of the world, to produce hybrids resembling in their stripes, not their own parents, but other species of the genus. To admit this view is, as it seems to me, to reject a real for an unreal, or at least for an unknown, cause. It makes the works of God a mere mockery and deception185; I would almost as soon believe with the old and ignorant cosmogonists, that fossil shells had never lived, but had been created in stone so as to mock the shells now living on the sea-shore.
SUMMARY.
Our ignorance of the laws of variation is profound. Not in one case out of a hundred can we pretend to assign any reason why this or that part differs, more or less, from the same part in the parents. But whenever we have the means of instituting a comparison, the same laws appear to have acted in producing the lesser differences between varieties of the same species, and the greater differences between species of the same genus. The external conditions of life, as climate and food, etc., seem to have induced some slight modifications. Habit in producing constitutional differences, and use in strengthening, and disuse in weakening and diminishing organs, seem to have been more potent186 in their effects. Homologous parts tend to vary in the same way, and homologous parts tend to cohere. Modifications in hard parts and in external parts sometimes affect softer and internal parts. When one part is largely developed, perhaps it tends to draw nourishment from the adjoining parts; and every part of the structure which can be saved without detriment187 to the individual, will be saved. Changes of structure at an early age will generally affect parts subsequently developed; and there are very many other correlations of growth, the nature of which we are utterly188 unable to understand. Multiple parts are variable in number and in structure, perhaps arising from such parts not having been closely specialised to any particular function, so that their modifications have not been closely checked by natural selection. It is probably from this same cause that organic beings low in the scale of nature are more variable than those which have their whole organisation more specialised, and are higher in the scale. Rudimentary organs, from being useless, will be disregarded by natural selection, and hence probably are variable. Specific characters--that is, the characters which have come to differ since the several species of the same genus branched off from a common parent--are more variable than generic characters, or those which have long been inherited, and have not differed within this same period. In these remarks we have referred to special parts or organs being still variable, because they have recently varied and thus come to differ; but we have also seen in the second Chapter that the same principle applies to the whole individual; for in a district where many species of any genus are found--that is, where there has been much former variation and differentiation189, or where the manufactory of new specific forms has been actively190 at work--there, on an average, we now find most varieties or incipient191 species. Secondary sexual characters are highly variable, and such characters differ much in the species of the same group. Variability in the same parts of the organisation has generally been taken advantage of in giving secondary sexual differences to the sexes of the same species, and specific differences to the several species of the same genus. Any part or organ developed to an extraordinary size or in an extraordinary manner, in comparison with the same part or organ in the allied species, must have gone through an extraordinary amount of modification since the genus arose; and thus we can understand why it should often still be variable in a much higher degree than other parts; for variation is a long-continued and slow process, and natural selection will in such cases not as yet have had time to overcome the tendency to further variability and to reversion to a less modified state. But when a species with any extraordinarily-developed organ has become the parent of many modified descendants--which on my view must be a very slow process, requiring a long lapse of time--in this case, natural selection may readily have succeeded in giving a fixed character to the organ, in however extraordinary a manner it may be developed. Species inheriting nearly the same constitution from a common parent and exposed to similar influences will naturally tend to present analogous variations, and these same species may occasionally revert to some of the characters of their ancient progenitors. Although new and important modifications may not arise from reversion and analogous variation, such modifications will add to the beautiful and harmonious192 diversity of nature.
Whatever the cause may be of each slight difference in the offspring from their parents--and a cause for each must exist--it is the steady accumulation, through natural selection, of such differences, when beneficial to the individual, that gives rise to all the more important modifications of structure, by which the innumerable beings on the face of this earth are enabled to struggle with each other, and the best adapted to survive.
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correlation
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| n.相互关系，相关，关连 | |
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correlations
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| 相互的关系( correlation的名词复数 ) | |
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rudiment
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| n.初步；初级；基本原理 | |
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generic
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| adj.一般的，普通的，共有的 | |
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analogous
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| adj.相似的；类似的 | |
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domestication
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| n.驯养，驯化 | |
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lesser
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| adj.次要的，较小的；adv.较小地，较少地 | |
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deviation
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| n.背离，偏离；偏差，偏向；离题 | |
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deviations
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| 背离，偏离( deviation的名词复数 )； 离经叛道的行为 | |
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10
cultivation
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| n.耕作，培养，栽培（法），养成 | |
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11
eminently
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| adv.突出地；显著地；不寻常地 | |
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12
susceptible
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| adj.过敏的，敏感的；易动感情的，易受感动的 | |
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13
functionally
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| adv.机能上地，官能地 | |
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14
affected
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| adj.不自然的，假装的 | |
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15
apparently
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| adv.显然地；表面上，似乎 | |
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16
essentially
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| adv.本质上，实质上，基本上 | |
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17
ass
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| n.驴；傻瓜，蠢笨的人 | |
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18
lurid
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| adj.可怕的；血红的；苍白的 | |
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19
indirectly
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| adv.间接地，不直接了当地 | |
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20
naturalist
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| n.博物学家(尤指直接观察动植物者) | |
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21
appreciable
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| adj.明显的，可见的，可估量的，可觉察的 | |
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22
alluded
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| 提及，暗指( allude的过去式和过去分词 ) | |
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23
modifications
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| n.缓和( modification的名词复数 )；限制；更改；改变 | |
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24
modification
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| n.修改，改进，缓和，减轻 | |
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25
prey
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| n.被掠食者，牺牲者，掠食；v.捕食，掠夺，折磨 | |
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26
ostrich
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| n.鸵鸟 | |
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27
progenitor
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| n.祖先，先驱 | |
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28
incapable
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| adj.无能力的，不能做某事的 | |
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29
anterior
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| adj.较早的；在前的 | |
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30
beetles
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|
| n.甲虫( beetle的名词复数 ) | |
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31
beetle
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| n.甲虫，近视眼的人 | |
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32
specimen
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| n.样本，标本 | |
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33
specimens
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|
| n.样品( specimen的名词复数 )；范例；（化验的）抽样；某种类型的人 | |
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34
relic
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|
| n.神圣的遗物，遗迹，纪念物 | |
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35
habitually
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| ad.习惯地，通常地 | |
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36
deficient
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|
| adj.不足的，不充份的，有缺陷的 | |
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37
progenitors
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|
| n.祖先( progenitor的名词复数 )；先驱；前辈；原本 | |
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38
remarkable
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|
| adj.显著的，异常的，非凡的，值得注意的 | |
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39
concealed
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|
| a.隐藏的，隐蔽的 | |
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40
lulls
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|
| n.间歇期（lull的复数形式）vt.使镇静，使安静（lull的第三人称单数形式） | |
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41
specially
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|
| adv.特定地；特殊地；明确地 | |
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42
necessitating
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| 使…成为必要，需要( necessitate的现在分词 ) | |
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43
perfectly
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|
| adv.完美地，无可非议地，彻底地 | |
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44
fully
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|
| adv.完全地，全部地，彻底地；充分地 | |
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45
mariners
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| 海员，水手（mariner的复数形式） | |
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46
wreck
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|
| n.失事，遇难；沉船；vt.(船等)失事，遇难 | |
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47
moles
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|
| 防波堤( mole的名词复数 )； 鼹鼠； 痣； 间谍 | |
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48
mole
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| n.胎块；痣；克分子 | |
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49
burrowing
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| v.挖掘（洞穴），挖洞( burrow的现在分词 )；翻寻 | |
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50
rodents
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|
| n.啮齿目动物( rodent的名词复数 ) | |
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51
rodent
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|
| n.啮齿动物；adj.啮齿目的 | |
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52
subterranean
|
|
| adj.地下的，地表下的 | |
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53
dissection
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|
| n.分析；解剖 | |
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54
membrane
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|
| n.薄膜，膜皮，羊皮纸 | |
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55
eyelids
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|
| n.眼睑( eyelid的名词复数 )；眼睛也不眨一下；不露声色；面不改色 | |
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56
asses
|
|
| n. 驴，愚蠢的人，臀部 adv. (常用作后置)用于贬损或骂人 | |
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57
crabs
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|
| n.蟹( crab的名词复数 )；阴虱寄生病；蟹肉v.捕蟹( crab的第三人称单数 ) | |
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58
remains
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|
| n.剩余物，残留物；遗体，遗迹 | |
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59
regained
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| 复得( regain的过去式和过去分词 )； 赢回； 重回； 复至某地 | |
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60
limestone
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| n.石灰石 | |
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61
caverns
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| 大山洞，大洞穴( cavern的名词复数 ) | |
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62
organisation
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| n.组织，安排，团体，有机休 | |
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63
affinities
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|
| n.密切关系( affinity的名词复数 )；亲近；（生性）喜爱；类同 | |
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64
allied
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| adj.协约国的；同盟国的 | |
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65
recesses
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|
| n.壁凹( recess的名词复数 )；（工作或业务活动的）中止或暂停期间；学校的课间休息；某物内部的凹形空间v.把某物放在墙壁的凹处( recess的第三人称单数 )；将（墙）做成凹形，在（墙）上做壁龛；休息，休会，休庭 | |
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66
twilight
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|
| n.暮光，黄昏；暮年，晚期，衰落时期 | |
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67
destined
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|
| adj.命中注定的；(for)以…为目的地的 | |
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68
obliterated
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|
| v.除去( obliterate的过去式和过去分词 )；涂去；擦掉；彻底破坏或毁灭 | |
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69
antennae
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|
| n.天线；触角 | |
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70
anomalous
|
|
| adj.反常的；不规则的 | |
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71
reptiles
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|
| n.爬行动物，爬虫( reptile的名词复数 ) | |
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72
wrecks
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|
| n.沉船( wreck的名词复数 )；（事故中）遭严重毁坏的汽车（或飞机等）；（身体或精神上）受到严重损伤的人；状况非常糟糕的车辆（或建筑物等）v.毁坏[毁灭]某物( wreck的第三人称单数 )；使（船舶）失事，使遇难，使下沉 | |
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73
abodes
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|
| 住所( abode的名词复数 )； 公寓； （在某地的）暂住； 逗留 | |
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74
hereditary
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|
| adj.遗传的，遗传性的，可继承的，世袭的 | |
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75
requisite
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|
| adj.需要的，必不可少的；n.必需品 | |
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76
germinate
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|
| v.发芽；发生；发展 | |
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77
descended
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|
| a.为...后裔的，出身于...的 | |
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78
temperate
|
|
| adj.温和的，温带的，自我克制的，不过分的 | |
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79
authentic
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| a.真的，真正的；可靠的，可信的，有根据的 | |
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80
latitudes
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| 纬度 | |
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81
positively
|
|
| adv.明确地，断然，坚决地；实在，确实 | |
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82
strictly
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|
| adv.严厉地，严格地；严密地 | |
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83
confinement
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|
| n.幽禁，拘留，监禁；分娩；限制，局限 | |
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84
mingled
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|
| 混合，混入( mingle的过去式和过去分词 )； 混进，与…交往[联系] | |
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85
grafted
|
|
| 移植( graft的过去式和过去分词 )； 嫁接； 使（思想、制度等）成为（…的一部份）； 植根 | |
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86
innate
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|
| adj.天生的，固有的，天赋的 | |
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87
flexibility
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|
| n.柔韧性，弹性，（光的）折射性，灵活性 | |
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88
rhinoceros
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| n.犀牛 | |
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89
mere
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|
| adj.纯粹的；仅仅，只不过 | |
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90
peculiar
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|
| adj.古怪的，异常的；特殊的，特有的 | |
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91
incessant
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|
| adj.不停的，连续的 | |
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92
encyclopaedias
|
|
| n.百科全书，大全( encyclopaedia的名词复数 ) | |
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93
treatise
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|
| n.专著；(专题)论文 | |
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94
treatises
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| n.专题著作，专题论文，专著( treatise的名词复数 ) | |
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95
survivors
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|
| 幸存者，残存者，生还者( survivor的名词复数 ) | |
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96
seedlings
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|
| n.刚出芽的幼苗( seedling的名词复数 ) | |
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97
seedling
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|
| n.秧苗，树苗 | |
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98
hardy
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|
| adj.勇敢的，果断的，吃苦的；耐寒的 | |
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99
solely
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| adv.仅仅，唯一地 | |
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100
embryo
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|
| n.胚胎，萌芽的事物 | |
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101
embryonic
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|
| adj.胚胎的 | |
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102
hind
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|
| adj.后面的，后部的 | |
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103
jaws
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|
| n.口部；嘴 | |
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104
jaw
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|
| n.颚，颌，说教，流言蜚语；v.喋喋不休，教训 | |
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105
cohere
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|
| vt.附着，连贯，一致 | |
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106
monstrous
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|
| adj.巨大的；恐怖的；可耻的，丢脸的 | |
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107
petals
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| n.花瓣( petal的名词复数 ) | |
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108
dermal
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| adj.皮肤的，真皮的 | |
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109
scaly
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| adj.鱼鳞状的；干燥粗糙的 | |
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110
abortion
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| n.流产，堕胎 | |
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111
countenances
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| n.面容( countenance的名词复数 )；表情；镇静；道义支持 | |
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112
densest
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| 密集的( dense的最高级 )； 密度大的； 愚笨的； （信息量大得）难理解的 | |
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113
nourishment
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| n.食物，营养品；营养情况 | |
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114
axis
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| n.轴，轴线，中心线；坐标轴，基准线 | |
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115
adherent
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| n.信徒，追随者，拥护者 | |
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116
aborted
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| adj.流产的，失败的v.（使）流产( abort的过去式和过去分词 )；（使）（某事物）中止；（因故障等而）（使）（飞机、宇宙飞船、导弹等）中断飞行；（使）（飞行任务等）中途失败 | |
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117
exterior
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|
| adj.外部的，外在的；表面的 | |
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118
advantageous
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| adj.有利的；有帮助的 | |
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119
entirely
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| ad.全部地，完整地；完全地，彻底地 | |
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120
wafted
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| v.吹送，飘送，（使）浮动( waft的过去式和过去分词 ) | |
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121
propounded
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|
| v.提出（问题、计划等）供考虑[讨论]，提议( propound的过去式和过去分词 ) | |
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122
fatten
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| v.使肥，变肥 | |
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123
nutritious
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|
| adj.有营养的，营养价值高的 | |
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124
foliage
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|
| n.叶子，树叶，簇叶 | |
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125
copious
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| adj.丰富的，大量的 | |
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126
atrophied
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|
| adj.萎缩的，衰退的v.（使）萎缩，（使）虚脱，（使）衰退( atrophy的过去式和过去分词 ) | |
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127
poultry
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| n.家禽，禽肉 | |
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128
botanists
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| n.植物学家，研究植物的人( botanist的名词复数 ) | |
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129
withdrawal
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| n.取回，提款；撤退，撤军；收回，撤销 | |
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130
merged
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| （使）混合( merge的过去式和过去分词 )； 相融； 融入； 渐渐消失在某物中 | |
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131
diminution
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|
| n.减少；变小 | |
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132
parasitic
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| adj.寄生的 | |
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133
carapace
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| n.（蟹或龟的）甲壳 | |
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134
prehensile
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| adj.（足等）适于抓握的 | |
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135
superfluous
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|
| adj.过多的，过剩的，多余的 | |
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136
decided
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| adj.决定了的，坚决的；明显的，明确的 | |
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137
naturalists
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| n.博物学家( naturalist的名词复数 )；（文学艺术的）自然主义者 | |
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138
diversified
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| adj.多样化的，多种经营的v.使多样化，多样化( diversify的过去式和过去分词 )；进入新的商业领域 | |
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139
recur
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| vi.复发，重现，再发生 | |
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140
briefly
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| adv.简单地，简短地 | |
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141
illustrates
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| 给…加插图( illustrate的第三人称单数 )； 说明； 表明； （用示例、图画等）说明 | |
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142
diversification
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| n.变化，多样化；多种经营 | |
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143
remarkably
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| ad.不同寻常地，相当地 | |
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144
presumption
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| n.推测，可能性，冒昧，放肆，[法律]推定 | |
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applied
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| adj.应用的；v.应用，适用 | |
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146
degenerated
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| 衰退，堕落，退化( degenerate的过去式和过去分词 ) | |
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147
prodigious
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| adj.惊人的，奇妙的；异常的；巨大的；庞大的 | |
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148
beak
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| n.鸟嘴，茶壶嘴，钩形鼻 | |
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extraordinarily
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| adv.格外地；极端地 | |
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fixed
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| adj.固定的，不变的，准备好的；(计算机)固定的 | |
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151
rejection
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| n.拒绝，被拒，抛弃，被弃 | |
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152
revert
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| v.恢复，复归，回到 | |
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153
physiological
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| adj.生理学的，生理学上的 | |
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154
considerably
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| adv.极大地；相当大地；在很大程度上 | |
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155
varied
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| adj.多样的，多变化的 | |
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156
rigid
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| adj.严格的，死板的；刚硬的，僵硬的 | |
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entail
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| vt.使承担，使成为必要，需要 | |
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158
joints
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| 接头( joint的名词复数 )； 关节； 公共场所（尤指价格低廉的饮食和娱乐场所） （非正式）； 一块烤肉 （英式英语） | |
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159
lapse
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| n.过失，流逝，失效，抛弃信仰，间隔；vi.堕落，停止，失效，流逝；vt.使失效 | |
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reverts
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| 恢复( revert的第三人称单数 )； 重提； 回到…上； 归还 | |
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161
possessed
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| adj.疯狂的；拥有的，占有的 | |
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aboriginal
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| adj.（指动植物）土生的，原产地的，土著的 | |
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turnip
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| n.萝卜，芜菁 | |
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formerly
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| adv.从前，以前 | |
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favourable
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| adj.赞成的，称赞的，有利的，良好的，顺利的 | |
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166
ascendancy
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| n.统治权，支配力量 | |
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mutual
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| adj.相互的，彼此的；共同的，共有的 | |
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168
tint
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| n.淡色，浅色；染发剂；vt.着以淡淡的颜色 | |
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undoubtedly
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| adv.确实地，无疑地 | |
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170
systematic
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| adj.有系统的，有计划的，有方法的 | |
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171
inquiries
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| n.调查( inquiry的名词复数 )；疑问；探究；打听 | |
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172
spinal
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| adj.针的，尖刺的，尖刺状突起的；adj.脊骨的，脊髓的 | |
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173
chestnut
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| n.栗树，栗子 | |
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174
sketch
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| n.草图；梗概；素描；v.素描；概述 | |
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implicitly
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| adv. 含蓄地, 暗中地, 毫不保留地 | |
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176
pony
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| adj.小型的；n.小马 | |
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177
spine
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| n.脊柱，脊椎；(动植物的)刺；书脊 | |
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178
ponies
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| 矮种马，小型马( pony的名词复数 )； ￡25 25 英镑 | |
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179
lanky
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| adj.瘦长的 | |
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180
mule
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| n.骡子，杂种，执拗的人 | |
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181
hybrids
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| n.杂交生成的生物体( hybrid的名词复数 )；杂交植物（或动物）；杂种；（不同事物的）混合物 | |
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182
hybrid
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| n.(动，植）杂种，混合物 | |
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183
mare
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| n.母马，母驴 | |
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184
geographical
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| adj.地理的；地区(性)的 | |
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185
deception
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| n.欺骗，欺诈；骗局，诡计 | |
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186
potent
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| adj.强有力的，有权势的；有效力的 | |
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187
detriment
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| n.损害；损害物，造成损害的根源 | |
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188
utterly
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| adv.完全地，绝对地 | |
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189
differentiation
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| n.区别，区分 | |
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190
actively
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| adv.积极地，勤奋地 | |
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191
incipient
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| adj.起初的，发端的，初期的 | |
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192
harmonious
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| adj.和睦的，调和的，和谐的，协调的 | |
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