Long before having arrived at this part of my work, a crowd of difficulties will have occurred to the reader. Some of them are so grave that to this day I can never reflect on them without being staggered; but, to the best of my judgment6, the greater number are only apparent, and those that are real are not, I think, fatal to my theory.
These difficulties and objections may be classed under the following heads:--
Firstly, why, if species have descended7 from other species by insensibly fine gradations, do we not everywhere see innumerable transitional forms? Why is not all nature in confusion instead of the species being, as we see them, well defined?
Secondly9, is it possible that an animal having, for instance, the structure and habits of a bat, could have been formed by the modification of some animal with wholly different habits? Can we believe that natural selection could produce, on the one hand, organs of trifling10 importance, such as the tail of a giraffe, which serves as a fly-flapper, and, on the other hand, organs of such wonderful structure, as the eye, of which we hardly as yet fully11 understand the inimitable perfection?
Thirdly, can instincts be acquired and modified through natural selection? What shall we say to so marvellous an instinct as that which leads the bee to make cells, which have practically anticipated the discoveries of profound mathematicians12?
Fourthly, how can we account for species, when crossed, being sterile13 and producing sterile offspring, whereas, when varieties are crossed, their fertility is unimpaired?
The two first heads shall be here discussed--Instinct and Hybridism14 in separate chapters.
ON THE ABSENCE OR RARITY OF TRANSITIONAL VARIETIES.
As natural selection acts solely15 by the preservation16 of profitable modifications17, each new form will tend in a fully-stocked country to take the place of, and finally to exterminate18, its own less improved parent or other less-favoured forms with which it comes into competition. Thus extinction19 and natural selection will, as we have seen, go hand in hand. Hence, if we look at each species as descended from some other unknown form, both the parent and all the transitional varieties will generally have been exterminated20 by the very process of formation and perfection of the new form.
But, as by this theory innumerable transitional forms must have existed, why do we not find them embedded21 in countless22 numbers in the crust of the earth? It will be much more convenient to discuss this question in the chapter on the Imperfection of the geological record; and I will here only state that I believe the answer mainly lies in the record being incomparably less perfect than is generally supposed; the imperfection of the record being chiefly due to organic beings not inhabiting profound depths of the sea, and to their remains23 being embedded and preserved to a future age only in masses of sediment24 sufficiently25 thick and extensive to withstand an enormous amount of future degradation26; and such fossiliferous masses can be accumulated only where much sediment is deposited on the shallow bed of the sea, whilst it slowly subsides27. These contingencies28 will concur29 only rarely, and after enormously long intervals30. Whilst the bed of the sea is stationary31 or is rising, or when very little sediment is being deposited, there will be blanks in our geological history. The crust of the earth is a vast museum; but the natural collections have been made only at intervals of time immensely remote.
But it may be urged that when several closely-allied32 species inhabit the same territory we surely ought to find at the present time many transitional forms. Let us take a simple case: in travelling from north to south over a continent, we generally meet at successive intervals with closely allied or representative species, evidently filling nearly the same place in the natural economy of the land. These representative species often meet and interlock; and as the one becomes rarer and rarer, the other becomes more and more frequent, till the one replaces the other. But if we compare these species where they intermingle, they are generally as absolutely distinct from each other in every detail of structure as are specimens34 taken from the metropolis35 inhabited by each. By my theory these allied species have descended from a common parent; and during the process of modification, each has become adapted to the conditions of life of its own region, and has supplanted37 and exterminated its original parent and all the transitional varieties between its past and present states. Hence we ought not to expect at the present time to meet with numerous transitional varieties in each region, though they must have existed there, and may be embedded there in a fossil condition. But in the intermediate region, having intermediate conditions of life, why do we not now find closely-linking intermediate varieties? This difficulty for a long time quite confounded me. But I think it can be in large part explained.
In the first place we should be extremely cautious in inferring, because an area is now continuous, that it has been continuous during a long period. Geology would lead us to believe that almost every continent has been broken up into islands even during the later tertiary periods; and in such islands distinct species might have been separately formed without the possibility of intermediate varieties existing in the intermediate zones. By changes in the form of the land and of climate, marine38 areas now continuous must often have existed within recent times in a far less continuous and uniform condition than at present. But I will pass over this way of escaping from the difficulty; for I believe that many perfectly39 defined species have been formed on strictly40 continuous areas; though I do not doubt that the formerly41 broken condition of areas now continuous has played an important part in the formation of new species, more especially with freely-crossing and wandering animals.
In looking at species as they are now distributed over a wide area, we generally find them tolerably numerous over a large territory, then becoming somewhat abruptly43 rarer and rarer on the confines, and finally disappearing. Hence the neutral territory between two representative species is generally narrow in comparison with the territory proper to each. We see the same fact in ascending44 mountains, and sometimes it is quite remarkable45 how abruptly, as Alph. De Candolle has observed, a common alpine46 species disappears. The same fact has been noticed by Forbes in sounding the depths of the sea with the dredge. To those who look at climate and the physical conditions of life as the all-important elements of distribution, these facts ought to cause surprise, as climate and height or depth graduate away insensibly. But when we bear in mind that almost every species, even in its metropolis, would increase immensely in numbers, were it not for other competing species; that nearly all either prey47 on or serve as prey for others; in short, that each organic being is either directly or indirectly48 related in the most important manner to other organic beings, we must see that the range of the inhabitants of any country by no means exclusively depends on insensibly changing physical conditions, but in large part on the presence of other species, on which it depends, or by which it is destroyed, or with which it comes into competition; and as these species are already defined objects (however they may have become so), not blending one into another by insensible gradations, the range of any one species, depending as it does on the range of others, will tend to be sharply defined. Moreover, each species on the confines of its range, where it exists in lessened50 numbers, will, during fluctuations51 in the number of its enemies or of its prey, or in the seasons, be extremely liable to utter extermination52; and thus its geographical53 range will come to be still more sharply defined.
If I am right in believing that allied or representative species, when inhabiting a continuous area, are generally so distributed that each has a wide range, with a comparatively narrow neutral territory between them, in which they become rather suddenly rarer and rarer; then, as varieties do not essentially54 differ from species, the same rule will probably apply to both; and if we in imagination adapt a varying species to a very large area, we shall have to adapt two varieties to two large areas, and a third variety to a narrow intermediate zone. The intermediate variety, consequently, will exist in lesser55 numbers from inhabiting a narrow and lesser area; and practically, as far as I can make out, this rule holds good with varieties in a state of nature. I have met with striking instances of the rule in the case of varieties intermediate between well-marked varieties in the genus Balanus. And it would appear from information given me by Mr. Watson, Dr. Asa Gray, and Mr. Wollaston, that generally when varieties intermediate between two other forms occur, they are much rarer numerically than the forms which they connect. Now, if we may trust these facts and inferences, and therefore conclude that varieties linking two other varieties together have generally existed in lesser numbers than the forms which they connect, then, I think, we can understand why intermediate varieties should not endure for very long periods;--why as a general rule they should be exterminated and disappear, sooner than the forms which they originally linked together.
For any form existing in lesser numbers would, as already remarked, run a greater chance of being exterminated than one existing in large numbers; and in this particular case the intermediate form would be eminently56 liable to the inroads of closely allied forms existing on both sides of it. But a far more important consideration, as I believe, is that, during the process of further modification, by which two varieties are supposed on my theory to be converted and perfected into two distinct species, the two which exist in larger numbers from inhabiting larger areas, will have a great advantage over the intermediate variety, which exists in smaller numbers in a narrow and intermediate zone. For forms existing in larger numbers will always have a better chance, within any given period, of presenting further favourable57 variations for natural selection to seize on, than will the rarer forms which exist in lesser numbers. Hence, the more common forms, in the race for life, will tend to beat and supplant36 the less common forms, for these will be more slowly modified and improved. It is the same principle which, as I believe, accounts for the common species in each country, as shown in the second chapter, presenting on an average a greater number of well-marked varieties than do the rarer species. I may illustrate58 what I mean by supposing three varieties of sheep to be kept, one adapted to an extensive mountainous region; a second to a comparatively narrow, hilly tract59; and a third to wide plains at the base; and that the inhabitants are all trying with equal steadiness and skill to improve their stocks by selection; the chances in this case will be strongly in favour of the great holders60 on the mountains or on the plains improving their breeds more quickly than the small holders on the intermediate narrow, hilly tract; and consequently the improved mountain or plain breed will soon take the place of the less improved hill breed; and thus the two breeds, which originally existed in greater numbers, will come into close contact with each other, without the interposition of the supplanted, intermediate hill-variety.
To sum up, I believe that species come to be tolerably well-defined objects, and do not at any one period present an inextricable chaos61 of varying and intermediate links: firstly, because new varieties are very slowly formed, for variation is a very slow process, and natural selection can do nothing until favourable variations chance to occur, and until a place in the natural polity of the country can be better filled by some modification of some one or more of its inhabitants. And such new places will depend on slow changes of climate, or on the occasional immigration of new inhabitants, and, probably, in a still more important degree, on some of the old inhabitants becoming slowly modified, with the new forms thus produced and the old ones acting62 and reacting on each other. So that, in any one region and at any one time, we ought only to see a few species presenting slight modifications of structure in some degree permanent; and this assuredly we do see.
Secondly, areas now continuous must often have existed within the recent period in isolated63 portions, in which many forms, more especially amongst the classes which unite for each birth and wander much, may have separately been rendered sufficiently distinct to rank as representative species. In this case, intermediate varieties between the several representative species and their common parent, must formerly have existed in each broken portion of the land, but these links will have been supplanted and exterminated during the process of natural selection, so that they will no longer exist in a living state.
Thirdly, when two or more varieties have been formed in different portions of a strictly continuous area, intermediate varieties will, it is probable, at first have been formed in the intermediate zones, but they will generally have had a short duration. For these intermediate varieties will, from reasons already assigned (namely from what we know of the actual distribution of closely allied or representative species, and likewise of acknowledged varieties), exist in the intermediate zones in lesser numbers than the varieties which they tend to connect. From this cause alone the intermediate varieties will be liable to accidental extermination; and during the process of further modification through natural selection, they will almost certainly be beaten and supplanted by the forms which they connect; for these from existing in greater numbers will, in the aggregate64, present more variation, and thus be further improved through natural selection and gain further advantages.
Lastly, looking not to any one time, but to all time, if my theory be true, numberless intermediate varieties, linking most closely all the species of the same group together, must assuredly have existed; but the very process of natural selection constantly tends, as has been so often remarked, to exterminate the parent forms and the intermediate links. Consequently evidence of their former existence could be found only amongst fossil remains, which are preserved, as we shall in a future chapter attempt to show, in an extremely imperfect and intermittent65 record.
ON THE ORIGIN AND TRANSITIONS OF ORGANIC BEINGS WITH PECULIAR66 HABITS AND STRUCTURE.
It has been asked by the opponents of such views as I hold, how, for instance, a land carnivorous animal could have been converted into one with aquatic67 habits; for how could the animal in its transitional state have subsisted68? It would be easy to show that within the same group carnivorous animals exist having every intermediate grade between truly aquatic and strictly terrestrial habits; and as each exists by a struggle for life, it is clear that each is well adapted in its habits to its place in nature. Look at the Mustela vison of North America, which has webbed feet and which resembles an otter69 in its fur, short legs, and form of tail; during summer this animal dives for and preys70 on fish, but during the long winter it leaves the frozen waters, and preys like other polecats on mice and land animals. If a different case had been taken, and it had been asked how an insectivorous quadruped could possibly have been converted into a flying bat, the question would have been far more difficult, and I could have given no answer. Yet I think such difficulties have very little weight.
Here, as on other occasions, I lie under a heavy disadvantage, for out of the many striking cases which I have collected, I can give only one or two instances of transitional habits and structures in closely allied species of the same genus; and of diversified habits, either constant or occasional, in the same species. And it seems to me that nothing less than a long list of such cases is sufficient to lessen49 the difficulty in any particular case like that of the bat.
Look at the family of squirrels; here we have the finest gradation from animals with their tails only slightly flattened71, and from others, as Sir J. Richardson has remarked, with the posterior part of their bodies rather wide and with the skin on their flanks rather full, to the so-called flying squirrels; and flying squirrels have their limbs and even the base of the tail united by a broad expanse of skin, which serves as a parachute and allows them to glide72 through the air to an astonishing distance from tree to tree. We cannot doubt that each structure is of use to each kind of squirrel in its own country, by enabling it to escape birds or beasts of prey, or to collect food more quickly, or, as there is reason to believe, by lessening73 the danger from occasional falls. But it does not follow from this fact that the structure of each squirrel is the best that it is possible to conceive under all natural conditions. Let the climate and vegetation change, let other competing rodents74 or new beasts of prey immigrate75, or old ones become modified, and all analogy would lead us to believe that some at least of the squirrels would decrease in numbers or become exterminated, unless they also became modified and improved in structure in a corresponding manner. Therefore, I can see no difficulty, more especially under changing conditions of life, in the continued preservation of individuals with fuller and fuller flank-membranes, each modification being useful, each being propagated, until by the accumulated effects of this process of natural selection, a perfect so-called flying squirrel was produced.
Now look at the Galeopithecus or flying lemur, which formerly was falsely ranked amongst bats. It has an extremely wide flank-membrane76, stretching from the corners of the jaw77 to the tail, and including the limbs and the elongated78 fingers: the flank membrane is, also, furnished with an extensor muscle. Although no graduated links of structure, fitted for gliding79 through the air, now connect the Galeopithecus with the other Lemuridae, yet I can see no difficulty in supposing that such links formerly existed, and that each had been formed by the same steps as in the case of the less perfectly gliding squirrels; and that each grade of structure had been useful to its possessor. Nor can I see any insuperable difficulty in further believing it possible that the membrane-connected fingers and fore5-arm of the Galeopithecus might be greatly lengthened80 by natural selection; and this, as far as the organs of flight are concerned, would convert it into a bat. In bats which have the wing-membrane extended from the top of the shoulder to the tail, including the hind81-legs, we perhaps see traces of an apparatus82 originally constructed for gliding through the air rather than for flight.
If about a dozen genera of birds had become extinct or were unknown, who would have ventured to have surmised83 that birds might have existed which used their wings solely as flappers, like the logger-headed duck (Micropterus of Eyton); as fins84 in the water and front legs on the land, like the penguin85; as sails, like the ostrich86; and functionally87 for no purpose, like the Apteryx. Yet the structure of each of these birds is good for it, under the conditions of life to which it is exposed, for each has to live by a struggle; but it is not necessarily the best possible under all possible conditions. It must not be inferred from these remarks that any of the grades of wing-structure here alluded88 to, which perhaps may all have resulted from disuse, indicate the natural steps by which birds have acquired their perfect power of flight; but they serve, at least, to show what diversified means of transition are possible.
Seeing that a few members of such water-breathing classes as the Crustacea and Mollusca are adapted to live on the land, and seeing that we have flying birds and mammals, flying insects of the most diversified types, and formerly had flying reptiles90, it is conceivable that flying-fish, which now glide far through the air, slightly rising and turning by the aid of their fluttering fins, might have been modified into perfectly winged animals. If this had been effected, who would have ever imagined that in an early transitional state they had been inhabitants of the open ocean, and had used their incipient91 organs of flight exclusively, as far as we know, to escape being devoured92 by other fish?
When we see any structure highly perfected for any particular habit, as the wings of a bird for flight, we should bear in mind that animals displaying early transitional grades of the structure will seldom continue to exist to the present day, for they will have been supplanted by the very process of perfection through natural selection. Furthermore, we may conclude that transitional grades between structures fitted for very different habits of life will rarely have been developed at an early period in great numbers and under many subordinate forms. Thus, to return to our imaginary illustration of the flying-fish, it does not seem probable that fishes capable of true flight would have been developed under many subordinate forms, for taking prey of many kinds in many ways, on the land and in the water, until their organs of flight had come to a high stage of perfection, so as to have given them a decided93 advantage over other animals in the battle for life. Hence the chance of discovering species with transitional grades of structure in a fossil condition will always be less, from their having existed in lesser numbers, than in the case of species with fully developed structures.
I will now give two or three instances of diversified and of changed habits in the individuals of the same species. When either case occurs, it would be easy for natural selection to fit the animal, by some modification of its structure, for its changed habits, or exclusively for one of its several different habits. But it is difficult to tell, and immaterial for us, whether habits generally change first and structure afterwards; or whether slight modifications of structure lead to changed habits; both probably often change almost simultaneously94. Of cases of changed habits it will suffice merely to allude89 to that of the many British insects which now feed on exotic plants, or exclusively on artificial substances. Of diversified habits innumerable instances could be given: I have often watched a tyrant96 flycatcher (Saurophagus sulphuratus) in South America, hovering97 over one spot and then proceeding98 to another, like a kestrel, and at other times standing99 stationary on the margin100 of water, and then dashing like a kingfisher at a fish. In our own country the larger titmouse (Parus major) may be seen climbing branches, almost like a creeper; it often, like a shrike, kills small birds by blows on the head; and I have many times seen and heard it hammering the seeds of the yew101 on a branch, and thus breaking them like a nuthatch. In North America the black bear was seen by Hearne swimming for hours with widely open mouth, thus catching102, like a whale, insects in the water. Even in so extreme a case as this, if the supply of insects were constant, and if better adapted competitors did not already exist in the country, I can see no difficulty in a race of bears being rendered, by natural selection, more and more aquatic in their structure and habits, with larger and larger mouths, till a creature was produced as monstrous103 as a whale.
As we sometimes see individuals of a species following habits widely different from those both of their own species and of the other species of the same genus, we might expect, on my theory, that such individuals would occasionally have given rise to new species, having anomalous104 habits, and with their structure either slightly or considerably105 modified from that of their proper type. And such instances do occur in nature. Can a more striking instance of adaptation be given than that of a woodpecker for climbing trees and for seizing insects in the chinks of the bark? Yet in North America there are woodpeckers which feed largely on fruit, and others with elongated wings which chase insects on the wing; and on the plains of La Plata, where not a tree grows, there is a woodpecker, which in every essential part of its organisation106, even in its colouring, in the harsh tone of its voice, and undulatory flight, told me plainly of its close blood-relationship to our common species; yet it is a woodpecker which never climbs a tree!
Petrels are the most aerial and oceanic of birds, yet in the quiet Sounds of Tierra del Fuego, the Puffinuria berardi, in its general habits, in its astonishing power of diving, its manner of swimming, and of flying when unwillingly107 it takes flight, would be mistaken by any one for an auk or grebe; nevertheless, it is essentially a petrel, but with many parts of its organisation profoundly modified. On the other hand, the acutest observer by examining the dead body of the water-ouzel would never have suspected its sub-aquatic habits; yet this anomalous member of the strictly terrestrial thrush family wholly subsists108 by diving,--grasping the stones with its feet and using its wings under water.
He who believes that each being has been created as we now see it, must occasionally have felt surprise when he has met with an animal having habits and structure not at all in agreement. What can be plainer than that the webbed feet of ducks and geese are formed for swimming? yet there are upland geese with webbed feet which rarely or never go near the water; and no one except Audubon has seen the frigate-bird, which has all its four toes webbed, alight on the surface of the sea. On the other hand, grebes and coots are eminently aquatic, although their toes are only bordered by membrane. What seems plainer than that the long toes of grallatores are formed for walking over swamps and floating plants, yet the water-hen is nearly as aquatic as the coot; and the landrail nearly as terrestrial as the quail109 or partridge. In such cases, and many others could be given, habits have changed without a corresponding change of structure. The webbed feet of the upland goose may be said to have become rudimentary in function, though not in structure. In the frigate-bird, the deeply-scooped membrane between the toes shows that structure has begun to change.
He who believes in separate and innumerable acts of creation will say, that in these cases it has pleased the Creator to cause a being of one type to take the place of one of another type; but this seems to me only restating the fact in dignified110 language. He who believes in the struggle for existence and in the principle of natural selection, will acknowledge that every organic being is constantly endeavouring to increase in numbers; and that if any one being vary ever so little, either in habits or structure, and thus gain an advantage over some other inhabitant of the country, it will seize on the place of that inhabitant, however different it may be from its own place. Hence it will cause him no surprise that there should be geese and frigate-birds with webbed feet, either living on the dry land or most rarely alighting on the water; that there should be long-toed corncrakes living in meadows instead of in swamps; that there should be woodpeckers where not a tree grows; that there should be diving thrushes, and petrels with the habits of auks.
ORGANS OF EXTREME PERFECTION AND COMPLICATION.
To suppose that the eye, with all its inimitable contrivances for adjusting the focus to different distances, for admitting different amounts of light, and for the correction of spherical111 and chromatic112 aberration113, could have been formed by natural selection, seems, I freely confess, absurd in the highest possible degree. Yet reason tells me, that if numerous gradations from a perfect and complex eye to one very imperfect and simple, each grade being useful to its possessor, can be shown to exist; if further, the eye does vary ever so slightly, and the variations be inherited, which is certainly the case; and if any variation or modification in the organ be ever useful to an animal under changing conditions of life, then the difficulty of believing that a perfect and complex eye could be formed by natural selection, though insuperable by our imagination, can hardly be considered real. How a nerve comes to be sensitive to light, hardly concerns us more than how life itself first originated; but I may remark that several facts make me suspect that any sensitive nerve may be rendered sensitive to light, and likewise to those coarser vibrations114 of the air which produce sound.
In looking for the gradations by which an organ in any species has been perfected, we ought to look exclusively to its lineal ancestors; but this is scarcely ever possible, and we are forced in each case to look to species of the same group, that is to the collateral115 descendants from the same original parent-form, in order to see what gradations are possible, and for the chance of some gradations having been transmitted from the earlier stages of descent, in an unaltered or little altered condition. Amongst existing Vertebrata, we find but a small amount of gradation in the structure of the eye, and from fossil species we can learn nothing on this head. In this great class we should probably have to descend8 far beneath the lowest known fossiliferous stratum117 to discover the earlier stages, by which the eye has been perfected.
In the Articulata we can commence a series with an optic nerve merely coated with pigment118, and without any other mechanism119; and from this low stage, numerous gradations of structure, branching off in two fundamentally different lines, can be shown to exist, until we reach a moderately high stage of perfection. In certain crustaceans120, for instance, there is a double cornea, the inner one divided into facets121, within each of which there is a lens-shaped swelling122. In other crustaceans the transparent123 cones124 which are coated by pigment, and which properly act only by excluding lateral116 pencils of light, are convex at their upper ends and must act by convergence; and at their lower ends there seems to be an imperfect vitreous substance. With these facts, here far too briefly125 and imperfectly given, which show that there is much graduated diversity in the eyes of living crustaceans, and bearing in mind how small the number of living animals is in proportion to those which have become extinct, I can see no very great difficulty (not more than in the case of many other structures) in believing that natural selection has converted the simple apparatus of an optic nerve merely coated with pigment and invested by transparent membrane, into an optical instrument as perfect as is possessed126 by any member of the great Articulate class.
He who will go thus far, if he find on finishing this treatise127 that large bodies of facts, otherwise inexplicable128, can be explained by the theory of descent, ought not to hesitate to go further, and to admit that a structure even as perfect as the eye of an eagle might be formed by natural selection, although in this case he does not know any of the transitional grades. His reason ought to conquer his imagination; though I have felt the difficulty far too keenly to be surprised at any degree of hesitation129 in extending the principle of natural selection to such startling lengths.
It is scarcely possible to avoid comparing the eye to a telescope. We know that this instrument has been perfected by the long-continued efforts of the highest human intellects; and we naturally infer that the eye has been formed by a somewhat analogous130 process. But may not this inference be presumptuous131? Have we any right to assume that the Creator works by intellectual powers like those of man? If we must compare the eye to an optical instrument, we ought in imagination to take a thick layer of transparent tissue, with a nerve sensitive to light beneath, and then suppose every part of this layer to be continually changing slowly in density132, so as to separate into layers of different densities133 and thicknesses, placed at different distances from each other, and with the surfaces of each layer slowly changing in form. Further we must suppose that there is a power always intently watching each slight accidental alteration134 in the transparent layers; and carefully selecting each alteration which, under varied135 circumstances, may in any way, or in any degree, tend to produce a distincter image. We must suppose each new state of the instrument to be multiplied by the million; and each to be preserved till a better be produced, and then the old ones to be destroyed. In living bodies, variation will cause the slight alterations136, generation will multiply them almost infinitely137, and natural selection will pick out with unerring skill each improvement. Let this process go on for millions on millions of years; and during each year on millions of individuals of many kinds; and may we not believe that a living optical instrument might thus be formed as superior to one of glass, as the works of the Creator are to those of man?
If it could be demonstrated that any complex organ existed, which could not possibly have been formed by numerous, successive, slight modifications, my theory would absolutely break down. But I can find out no such case. No doubt many organs exist of which we do not know the transitional grades, more especially if we look to much-isolated species, round which, according to my theory, there has been much extinction. Or again, if we look to an organ common to all the members of a large class, for in this latter case the organ must have been first formed at an extremely remote period, since which all the many members of the class have been developed; and in order to discover the early transitional grades through which the organ has passed, we should have to look to very ancient ancestral forms, long since become extinct.
We should be extremely cautious in concluding that an organ could not have been formed by transitional gradations of some kind. Numerous cases could be given amongst the lower animals of the same organ performing at the same time wholly distinct functions; thus the alimentary138 canal respires, digests, and excretes in the larva of the dragon-fly and in the fish Cobites. In the Hydra139, the animal may be turned inside out, and the exterior140 surface will then digest and the stomach respire. In such cases natural selection might easily specialise, if any advantage were thus gained, a part or organ, which had performed two functions, for one function alone, and thus wholly change its nature by insensible steps. Two distinct organs sometimes perform simultaneously the same function in the same individual; to give one instance, there are fish with gills or branchiae that breathe the air dissolved in the water, at the same time that they breathe free air in their swimbladders, this latter organ having a ductus pneumaticus for its supply, and being divided by highly vascular142 partitions. In these cases, one of the two organs might with ease be modified and perfected so as to perform all the work by itself, being aided during the process of modification by the other organ; and then this other organ might be modified for some other and quite distinct purpose, or be quite obliterated143.
The illustration of the swimbladder in fishes is a good one, because it shows us clearly the highly important fact that an organ originally constructed for one purpose, namely flotation, may be converted into one for a wholly different purpose, namely respiration144. The swimbladder has, also, been worked in as an accessory to the auditory organs of certain fish, or, for I do not know which view is now generally held, a part of the auditory apparatus has been worked in as a complement145 to the swimbladder. All physiologists146 admit that the swimbladder is homologous, or "ideally similar," in position and structure with the lungs of the higher vertebrate animals: hence there seems to me to be no great difficulty in believing that natural selection has actually converted a swimbladder into a lung, or organ used exclusively for respiration.
I can, indeed, hardly doubt that all vertebrate animals having true lungs have descended by ordinary generation from an ancient prototype, of which we know nothing, furnished with a floating apparatus or swimbladder. We can thus, as I infer from Professor Owen's interesting description of these parts, understand the strange fact that every particle of food and drink which we swallow has to pass over the orifice of the trachea, with some risk of falling into the lungs, notwithstanding the beautiful contrivance by which the glottis is closed. In the higher Vertebrata the branchiae have wholly disappeared--the slits147 on the sides of the neck and the loop-like course of the arteries148 still marking in the embryo149 their former position. But it is conceivable that the now utterly150 lost branchiae might have been gradually worked in by natural selection for some quite distinct purpose: in the same manner as, on the view entertained by some naturalists151 that the branchiae and dorsal153 scales of Annelids are homologous with the wings and wing-covers of insects, it is probable that organs which at a very ancient period served for respiration have been actually converted into organs of flight.
In considering transitions of organs, it is so important to bear in mind the probability of conversion154 from one function to another, that I will give one more instance. Pedunculated cirripedes have two minute folds of skin, called by me the ovigerous frena, which serve, through the means of a sticky secretion155, to retain the eggs until they are hatched within the sack. These cirripedes have no branchiae, the whole surface of the body and sack, including the small frena, serving for respiration. The Balanidae or sessile cirripedes, on the other hand, have no ovigerous frena, the eggs lying loose at the bottom of the sack, in the well-enclosed shell; but they have large folded branchiae. Now I think no one will dispute that the ovigerous frena in the one family are strictly homologous with the branchiae of the other family; indeed, they graduate into each other. Therefore I do not doubt that little folds of skin, which originally served as ovigerous frena, but which, likewise, very slightly aided the act of respiration, have been gradually converted by natural selection into branchiae, simply through an increase in their size and the obliteration156 of their adhesive157 glands158. If all pedunculated cirripedes had become extinct, and they have already suffered far more extinction than have sessile cirripedes, who would ever have imagined that the branchiae in this latter family had originally existed as organs for preventing the ova from being washed out of the sack?
Although we must be extremely cautious in concluding that any organ could not possibly have been produced by successive transitional gradations, yet, undoubtedly160, grave cases of difficulty occur, some of which will be discussed in my future work.
One of the gravest is that of neuter insects, which are often very differently constructed from either the males or fertile females; but this case will be treated of in the next chapter. The electric organs of fishes offer another case of special difficulty; it is impossible to conceive by what steps these wondrous161 organs have been produced; but, as Owen and others have remarked, their intimate structure closely resembles that of common muscle; and as it has lately been shown that Rays have an organ closely analogous to the electric apparatus, and yet do not, as Matteuchi asserts, discharge any electricity, we must own that we are far too ignorant to argue that no transition of any kind is possible.
The electric organs offer another and even more serious difficulty; for they occur in only about a dozen fishes, of which several are widely remote in their affinities162. Generally when the same organ appears in several members of the same class, especially if in members having very different habits of life, we may attribute its presence to inheritance from a common ancestor; and its absence in some of the members to its loss through disuse or natural selection. But if the electric organs had been inherited from one ancient progenitor163 thus provided, we might have expected that all electric fishes would have been specially42 related to each other. Nor does geology at all lead to the belief that formerly most fishes had electric organs, which most of their modified descendants have lost. The presence of luminous164 organs in a few insects, belonging to different families and orders, offers a parallel case of difficulty. Other cases could be given; for instance in plants, the very curious contrivance of a mass of pollen165-grains, borne on a foot-stalk with a sticky gland159 at the end, is the same in Orchis and Asclepias,--genera almost as remote as possible amongst flowering plants. In all these cases of two very distinct species furnished with apparently166 the same anomalous organ, it should be observed that, although the general appearance and function of the organ may be the same, yet some fundamental difference can generally be detected. I am inclined to believe that in nearly the same way as two men have sometimes independently hit on the very same invention, so natural selection, working for the good of each being and taking advantage of analogous variations, has sometimes modified in very nearly the same manner two parts in two organic beings, which owe but little of their structure in common to inheritance from the same ancestor.
Although in many cases it is most difficult to conjecture167 by what transitions an organ could have arrived at its present state; yet, considering that the proportion of living and known forms to the extinct and unknown is very small, I have been astonished how rarely an organ can be named, towards which no transitional grade is known to lead. The truth of this remark is indeed shown by that old canon in natural history of "Natura non facit saltum." We meet with this admission in the writings of almost every experienced naturalist152; or, as Milne Edwards has well expressed it, nature is prodigal168 in variety, but niggard in innovation. Why, on the theory of Creation, should this be so? Why should all the parts and organs of many independent beings, each supposed to have been separately created for its proper place in nature, be so invariably linked together by graduated steps? Why should not Nature have taken a leap from structure to structure? On the theory of natural selection, we can clearly understand why she should not; for natural selection can act only by taking advantage of slight successive variations; she can never take a leap, but must advance by the shortest and slowest steps.
ORGANS OF LITTLE APPARENT IMPORTANCE.
As natural selection acts by life and death,--by the preservation of individuals with any favourable variation, and by the destruction of those with any unfavourable deviation169 of structure,--I have sometimes felt much difficulty in understanding the origin of simple parts, of which the importance does not seem sufficient to cause the preservation of successively varying individuals. I have sometimes felt as much difficulty, though of a very different kind, on this head, as in the case of an organ as perfect and complex as the eye.
In the first place, we are much too ignorant in regard to the whole economy of any one organic being, to say what slight modifications would be of importance or not. In a former chapter I have given instances of most trifling characters, such as the down on fruit and the colour of the flesh, which, from determining the attacks of insects or from being correlated with constitutional differences, might assuredly be acted on by natural selection. The tail of the giraffe looks like an artificially constructed fly-flapper; and it seems at first incredible that this could have been adapted for its present purpose by successive slight modifications, each better and better, for so trifling an object as driving away flies; yet we should pause before being too positive even in this case, for we know that the distribution and existence of cattle and other animals in South America absolutely depends on their power of resisting the attacks of insects: so that individuals which could by any means defend themselves from these small enemies, would be able to range into new pastures and thus gain a great advantage. It is not that the larger quadrupeds are actually destroyed (except in some rare cases) by the flies, but they are incessantly170 harassed171 and their strength reduced, so that they are more subject to disease, or not so well enabled in a coming dearth172 to search for food, or to escape from beasts of prey.
Organs now of trifling importance have probably in some cases been of high importance to an early progenitor, and, after having been slowly perfected at a former period, have been transmitted in nearly the same state, although now become of very slight use; and any actually injurious deviations173 in their structure will always have been checked by natural selection. Seeing how important an organ of locomotion174 the tail is in most aquatic animals, its general presence and use for many purposes in so many land animals, which in their lungs or modified swim-bladders betray their aquatic origin, may perhaps be thus accounted for. A well-developed tail having been formed in an aquatic animal, it might subsequently come to be worked in for all sorts of purposes, as a fly-flapper, an organ of prehension, or as an aid in turning, as with the dog, though the aid must be slight, for the hare, with hardly any tail, can double quickly enough.
In the second place, we may sometimes attribute importance to characters which are really of very little importance, and which have originated from quite secondary causes, independently of natural selection. We should remember that climate, food, etc., probably have some little direct influence on the organisation; that characters reappear from the law of reversion; that correlation175 of growth will have had a most important influence in modifying various structures; and finally, that sexual selection will often have largely modified the external characters of animals having a will, to give one male an advantage in fighting with another or in charming the females. Moreover when a modification of structure has primarily arisen from the above or other unknown causes, it may at first have been of no advantage to the species, but may subsequently have been taken advantage of by the descendants of the species under new conditions of life and with newly acquired habits.
To give a few instances to illustrate these latter remarks. If green woodpeckers alone had existed, and we did not know that there were many black and pied kinds, I dare say that we should have thought that the green colour was a beautiful adaptation to hide this tree-frequenting bird from its enemies; and consequently that it was a character of importance and might have been acquired through natural selection; as it is, I have no doubt that the colour is due to some quite distinct cause, probably to sexual selection. A trailing bamboo in the Malay Archipelago climbs the loftiest trees by the aid of exquisitely176 constructed hooks clustered around the ends of the branches, and this contrivance, no doubt, is of the highest service to the plant; but as we see nearly similar hooks on many trees which are not climbers, the hooks on the bamboo may have arisen from unknown laws of growth, and have been subsequently taken advantage of by the plant undergoing further modification and becoming a climber. The naked skin on the head of a vulture is generally looked at as a direct adaptation for wallowing in putridity178; and so it may be, or it may possibly be due to the direct action of putrid177 matter; but we should be very cautious in drawing any such inference, when we see that the skin on the head of the clean-feeding male turkey is likewise naked. The sutures in the skulls179 of young mammals have been advanced as a beautiful adaptation for aiding parturition180, and no doubt they facilitate, or may be indispensable for this act; but as sutures occur in the skulls of young birds and reptiles, which have only to escape from a broken egg, we may infer that this structure has arisen from the laws of growth, and has been taken advantage of in the parturition of the higher animals.
We are profoundly ignorant of the causes producing slight and unimportant variations; and we are immediately made conscious of this by reflecting on the differences in the breeds of our domesticated181 animals in different countries,--more especially in the less civilized182 countries where there has been but little artificial selection. Careful observers are convinced that a damp climate affects the growth of the hair, and that with the hair the horns are correlated. Mountain breeds always differ from lowland breeds; and a mountainous country would probably affect the hind limbs from exercising them more, and possibly even the form of the pelvis; and then by the law of homologous variation, the front limbs and even the head would probably be affected183. The shape, also, of the pelvis might affect by pressure the shape of the head of the young in the womb. The laborious184 breathing necessary in high regions would, we have some reason to believe, increase the size of the chest; and again correlation would come into play. Animals kept by savages185 in different countries often have to struggle for their own subsistence, and would be exposed to a certain extent to natural selection, and individuals with slightly different constitutions would succeed best under different climates; and there is reason to believe that constitution and colour are correlated. A good observer, also, states that in cattle susceptibility to the attacks of flies is correlated with colour, as is the liability to be poisoned by certain plants; so that colour would be thus subjected to the action of natural selection. But we are far too ignorant to speculate on the relative importance of the several known and unknown laws of variation; and I have here alluded to them only to show that, if we are unable to account for the characteristic differences of our domestic breeds, which nevertheless we generally admit to have arisen through ordinary generation, we ought not to lay too much stress on our ignorance of the precise cause of the slight analogous differences between species. I might have adduced for this same purpose the differences between the races of man, which are so strongly marked; I may add that some little light can apparently be thrown on the origin of these differences, chiefly through sexual selection of a particular kind, but without here entering on copious187 details my reasoning would appear frivolous188.
The foregoing remarks lead me to say a few words on the protest lately made by some naturalists, against the utilitarian189 doctrine190 that every detail of structure has been produced for the good of its possessor. They believe that very many structures have been created for beauty in the eyes of man, or for mere95 variety. This doctrine, if true, would be absolutely fatal to my theory. Yet I fully admit that many structures are of no direct use to their possessors. Physical conditions probably have had some little effect on structure, quite independently of any good thus gained. Correlation of growth has no doubt played a most important part, and a useful modification of one part will often have entailed191 on other parts diversified changes of no direct use. So again characters which formerly were useful, or which formerly had arisen from correlation of growth, or from other unknown cause, may reappear from the law of reversion, though now of no direct use. The effects of sexual selection, when displayed in beauty to charm the females, can be called useful only in rather a forced sense. But by far the most important consideration is that the chief part of the organisation of every being is simply due to inheritance; and consequently, though each being assuredly is well fitted for its place in nature, many structures now have no direct relation to the habits of life of each species. Thus, we can hardly believe that the webbed feet of the upland goose or of the frigate-bird are of special use to these birds; we cannot believe that the same bones in the arm of the monkey, in the fore leg of the horse, in the wing of the bat, and in the flipper192 of the seal, are of special use to these animals. We may safely attribute these structures to inheritance. But to the progenitor of the upland goose and of the frigate-bird, webbed feet no doubt were as useful as they now are to the most aquatic of existing birds. So we may believe that the progenitor of the seal had not a flipper, but a foot with five toes fitted for walking or grasping; and we may further venture to believe that the several bones in the limbs of the monkey, horse, and bat, which have been inherited from a common progenitor, were formerly of more special use to that progenitor, or its progenitors193, than they now are to these animals having such widely diversified habits. Therefore we may infer that these several bones might have been acquired through natural selection, subjected formerly, as now, to the several laws of inheritance, reversion, correlation of growth, etc. Hence every detail of structure in every living creature (making some little allowance for the direct action of physical conditions) may be viewed, either as having been of special use to some ancestral form, or as being now of special use to the descendants of this form--either directly, or indirectly through the complex laws of growth.
Natural selection cannot possibly produce any modification in any one species exclusively for the good of another species; though throughout nature one species incessantly takes advantage of, and profits by, the structure of another. But natural selection can and does often produce structures for the direct injury of other species, as we see in the fang194 of the adder141, and in the ovipositor of the ichneumon, by which its eggs are deposited in the living bodies of other insects. If it could be proved that any part of the structure of any one species had been formed for the exclusive good of another species, it would annihilate195 my theory, for such could not have been produced through natural selection. Although many statements may be found in works on natural history to this effect, I cannot find even one which seems to me of any weight. It is admitted that the rattlesnake has a poison-fang for its own defence and for the destruction of its prey; but some authors suppose that at the same time this snake is furnished with a rattle196 for its own injury, namely, to warn its prey to escape. I would almost as soon believe that the cat curls the end of its tail when preparing to spring, in order to warn the doomed197 mouse. But I have not space here to enter on this and other such cases.
Natural selection will never produce in a being anything injurious to itself, for natural selection acts solely by and for the good of each. No organ will be formed, as Paley has remarked, for the purpose of causing pain or for doing an injury to its possessor. If a fair balance be struck between the good and evil caused by each part, each will be found on the whole advantageous198. After the lapse199 of time, under changing conditions of life, if any part comes to be injurious, it will be modified; or if it be not so, the being will become extinct, as myriads200 have become extinct.
Natural selection tends only to make each organic being as perfect as, or slightly more perfect than, the other inhabitants of the same country with which it has to struggle for existence. And we see that this is the degree of perfection attained201 under nature. The endemic productions of New Zealand, for instance, are perfect one compared with another; but they are now rapidly yielding before the advancing legions of plants and animals introduced from Europe. Natural selection will not produce absolute perfection, nor do we always meet, as far as we can judge, with this high standard under nature. The correction for the aberration of light is said, on high authority, not to be perfect even in that most perfect organ, the eye. If our reason leads us to admire with enthusiasm a multitude of inimitable contrivances in nature, this same reason tells us, though we may easily err33 on both sides, that some other contrivances are less perfect. Can we consider the sting of the wasp202 or of the bee as perfect, which, when used against many attacking animals, cannot be withdrawn203, owing to the backward serratures, and so inevitably204 causes the death of the insect by tearing out its viscera?
If we look at the sting of the bee, as having originally existed in a remote progenitor as a boring and serrated instrument, like that in so many members of the same great order, and which has been modified but not perfected for its present purpose, with the poison originally adapted to cause galls205 subsequently intensified206, we can perhaps understand how it is that the use of the sting should so often cause the insect's own death: for if on the whole the power of stinging be useful to the community, it will fulfil all the requirements of natural selection, though it may cause the death of some few members. If we admire the truly wonderful power of scent1 by which the males of many insects find their females, can we admire the production for this single purpose of thousands of drones, which are utterly useless to the community for any other end, and which are ultimately slaughtered207 by their industrious208 and sterile sisters? It may be difficult, but we ought to admire the savage186 instinctive209 hatred210 of the queen-bee, which urges her instantly to destroy the young queens her daughters as soon as born, or to perish herself in the combat; for undoubtedly this is for the good of the community; and maternal211 love or maternal hatred, though the latter fortunately is most rare, is all the same to the inexorable principle of natural selection. If we admire the several ingenious contrivances, by which the flowers of the orchis and of many other plants are fertilised through insect agency, can we consider as equally perfect the elaboration by our fir-trees of dense212 clouds of pollen, in order that a few granules may be wafted213 by a chance breeze on to the ovules?
SUMMARY OF CHAPTER.
We have in this chapter discussed some of the difficulties and objections which may be urged against my theory. Many of them are very grave; but I think that in the discussion light has been thrown on several facts, which on the theory of independent acts of creation are utterly obscure. We have seen that species at any one period are not indefinitely variable, and are not linked together by a multitude of intermediate gradations, partly because the process of natural selection will always be very slow, and will act, at any one time, only on a very few forms; and partly because the very process of natural selection almost implies the continual supplanting214 and extinction of preceding and intermediate gradations. Closely allied species, now living on a continuous area, must often have been formed when the area was not continuous, and when the conditions of life did not insensibly graduate away from one part to another. When two varieties are formed in two districts of a continuous area, an intermediate variety will often be formed, fitted for an intermediate zone; but from reasons assigned, the intermediate variety will usually exist in lesser numbers than the two forms which it connects; consequently the two latter, during the course of further modification, from existing in greater numbers, will have a great advantage over the less numerous intermediate variety, and will thus generally succeed in supplanting and exterminating215 it.
We have seen in this chapter how cautious we should be in concluding that the most different habits of life could not graduate into each other; that a bat, for instance, could not have been formed by natural selection from an animal which at first could only glide through the air.
We have seen that a species may under new conditions of life change its habits, or have diversified habits, with some habits very unlike those of its nearest congeners. Hence we can understand, bearing in mind that each organic being is trying to live wherever it can live, how it has arisen that there are upland geese with webbed feet, ground woodpeckers, diving thrushes, and petrels with the habits of auks.
Although the belief that an organ so perfect as the eye could have been formed by natural selection, is more than enough to stagger any one; yet in the case of any organ, if we know of a long series of gradations in complexity216, each good for its possessor, then, under changing conditions of life, there is no logical impossibility in the acquirement of any conceivable degree of perfection through natural selection. In the cases in which we know of no intermediate or transitional states, we should be very cautious in concluding that none could have existed, for the homologies of many organs and their intermediate states show that wonderful metamorphoses in function are at least possible. For instance, a swim-bladder has apparently been converted into an air-breathing lung. The same organ having performed simultaneously very different functions, and then having been specialised for one function; and two very distinct organs having performed at the same time the same function, the one having been perfected whilst aided by the other, must often have largely facilitated transitions.
We are far too ignorant, in almost every case, to be enabled to assert that any part or organ is so unimportant for the welfare of a species, that modifications in its structure could not have been slowly accumulated by means of natural selection. But we may confidently believe that many modifications, wholly due to the laws of growth, and at first in no way advantageous to a species, have been subsequently taken advantage of by the still further modified descendants of this species. We may, also, believe that a part formerly of high importance has often been retained (as the tail of an aquatic animal by its terrestrial descendants), though it has become of such small importance that it could not, in its present state, have been acquired by natural selection,--a power which acts solely by the preservation of profitable variations in the struggle for life.
Natural selection will produce nothing in one species for the exclusive good or injury of another; though it may well produce parts, organs, and excretions highly useful or even indispensable, or highly injurious to another species, but in all cases at the same time useful to the owner. Natural selection in each well-stocked country, must act chiefly through the competition of the inhabitants one with another, and consequently will produce perfection, or strength in the battle for life, only according to the standard of that country. Hence the inhabitants of one country, generally the smaller one, will often yield, as we see they do yield, to the inhabitants of another and generally larger country. For in the larger country there will have existed more individuals, and more diversified forms, and the competition will have been severer, and thus the standard of perfection will have been rendered higher. Natural selection will not necessarily produce absolute perfection; nor, as far as we can judge by our limited faculties217, can absolute perfection be everywhere found.
On the theory of natural selection we can clearly understand the full meaning of that old canon in natural history, "Natura non facit saltum." This canon, if we look only to the present inhabitants of the world, is not strictly correct, but if we include all those of past times, it must by my theory be strictly true.
It is generally acknowledged that all organic beings have been formed on two great laws--Unity of Type, and the Conditions of Existence. By unity of type is meant that fundamental agreement in structure, which we see in organic beings of the same class, and which is quite independent of their habits of life. On my theory, unity of type is explained by unity of descent. The expression of conditions of existence, so often insisted on by the illustrious Cuvier, is fully embraced by the principle of natural selection. For natural selection acts by either now adapting the varying parts of each being to its organic and inorganic218 conditions of life; or by having adapted them during long-past periods of time: the adaptations being aided in some cases by use and disuse, being slightly affected by the direct action of the external conditions of life, and being in all cases subjected to the several laws of growth. Hence, in fact, the law of the Conditions of Existence is the higher law; as it includes, through the inheritance of former adaptations, that of Unity of Type.
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scent
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| n.气味，香味，香水，线索，嗅觉；v.嗅，发觉 | |
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modification
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| n.修改，改进，缓和，减轻 | |
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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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unity
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| n.团结，联合，统一；和睦，协调 | |
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fore
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| adv.在前面；adj.先前的；在前部的；n.前部 | |
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judgment
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| n.审判；判断力，识别力，看法，意见 | |
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descended
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| a.为...后裔的，出身于...的 | |
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descend
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| vt./vi.传下来，下来，下降 | |
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secondly
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| adv.第二，其次 | |
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trifling
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| adj.微不足道的；没什么价值的 | |
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fully
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| adv.完全地，全部地，彻底地；充分地 | |
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mathematicians
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| 数学家( mathematician的名词复数 ) | |
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sterile
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| adj.不毛的，不孕的，无菌的，枯燥的，贫瘠的 | |
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hybridism
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| n.杂种，杂交 | |
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solely
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| adv.仅仅，唯一地 | |
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preservation
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| n.保护，维护，保存，保留，保持 | |
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modifications
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| n.缓和( modification的名词复数 )；限制；更改；改变 | |
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exterminate
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| v.扑灭，消灭，根绝 | |
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extinction
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| n.熄灭，消亡，消灭，灭绝，绝种 | |
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exterminated
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| v.消灭，根绝( exterminate的过去式和过去分词 ) | |
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embedded
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| a.扎牢的 | |
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countless
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| adj.无数的，多得不计其数的 | |
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remains
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| n.剩余物，残留物；遗体，遗迹 | |
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sediment
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| n.沉淀，沉渣，沉积(物) | |
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sufficiently
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| adv.足够地，充分地 | |
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degradation
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| n.降级；低落；退化；陵削；降解；衰变 | |
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subsides
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| v.（土地）下陷（因在地下采矿）( subside的第三人称单数 )；减弱；下降至较低或正常水平；一下子坐在椅子等上 | |
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contingencies
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| n.偶然发生的事故，意外事故( contingency的名词复数 )；以备万一 | |
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concur
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| v.同意，意见一致，互助，同时发生 | |
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intervals
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| n.[军事]间隔( interval的名词复数 )；间隔时间；[数学]区间；（戏剧、电影或音乐会的）幕间休息 | |
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stationary
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| adj.固定的，静止不动的 | |
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allied
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| adj.协约国的；同盟国的 | |
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err
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| vi.犯错误，出差错 | |
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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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metropolis
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| n.首府；大城市 | |
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supplant
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| vt.排挤；取代 | |
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supplanted
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| 把…排挤掉，取代( supplant的过去式和过去分词 ) | |
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38
marine
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| adj.海的；海生的；航海的；海事的；n.水兵 | |
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39
perfectly
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| adv.完美地，无可非议地，彻底地 | |
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40
strictly
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| adv.严厉地，严格地；严密地 | |
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41
formerly
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| adv.从前，以前 | |
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42
specially
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| adv.特定地；特殊地；明确地 | |
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43
abruptly
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| adv.突然地，出其不意地 | |
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44
ascending
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| adj.上升的，向上的 | |
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45
remarkable
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| adj.显著的，异常的，非凡的，值得注意的 | |
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46
alpine
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| adj.高山的；n.高山植物 | |
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47
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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48
indirectly
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| adv.间接地，不直接了当地 | |
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49
lessen
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| vt.减少，减轻；缩小 | |
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50
lessened
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| 减少的，减弱的 | |
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51
fluctuations
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| 波动，涨落，起伏( fluctuation的名词复数 ) | |
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52
extermination
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| n.消灭，根绝 | |
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53
geographical
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| adj.地理的；地区(性)的 | |
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54
essentially
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| adv.本质上，实质上，基本上 | |
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55
lesser
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| adj.次要的，较小的；adv.较小地，较少地 | |
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56
eminently
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| adv.突出地；显著地；不寻常地 | |
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57
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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58
illustrate
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| v.举例说明，阐明；图解，加插图 | |
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59
tract
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| n.传单，小册子，大片(土地或森林) | |
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60
holders
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| 支持物( holder的名词复数 )； 持有者； （支票等）持有人； 支托（或握持）…之物 | |
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61
chaos
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| n.混乱，无秩序 | |
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62
acting
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| n.演戏，行为，假装；adj.代理的，临时的，演出用的 | |
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63
isolated
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| adj.与世隔绝的 | |
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64
aggregate
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| adj.总计的，集合的；n.总数；v.合计；集合 | |
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65
intermittent
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| adj.间歇的，断断续续的 | |
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66
peculiar
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| adj.古怪的，异常的；特殊的，特有的 | |
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67
aquatic
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| adj.水生的，水栖的 | |
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68
subsisted
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| v.（靠很少的钱或食物）维持生活，生存下去( subsist的过去式和过去分词 ) | |
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69
otter
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| n.水獭 | |
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70
preys
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| v.掠食( prey的第三人称单数 )；掠食；折磨；（人）靠欺诈为生 | |
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71
flattened
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| [医]（水）平扁的，弄平的 | |
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72
glide
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| n./v.溜，滑行；(时间)消逝 | |
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73
lessening
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| 减轻，减少，变小 | |
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74
rodents
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|
| n.啮齿目动物( rodent的名词复数 ) | |
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75
immigrate
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| v.（从外国）移来，移居入境 | |
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76
membrane
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| n.薄膜，膜皮，羊皮纸 | |
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77
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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78
elongated
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| v.延长，加长( elongate的过去式和过去分词 ) | |
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79
gliding
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| v. 滑翔 adj. 滑动的 | |
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80
lengthened
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| （时间或空间）延长，伸长( lengthen的过去式和过去分词 ) | |
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81
hind
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| adj.后面的，后部的 | |
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82
apparatus
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| n.装置，器械；器具，设备 | |
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83
surmised
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|
| v.臆测，推断( surmise的过去式和过去分词 )；揣测；猜想 | |
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84
fins
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| [医]散热片；鱼鳍；飞边；鸭掌 | |
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85
penguin
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| n.企鹅 | |
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86
ostrich
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| n.鸵鸟 | |
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87
functionally
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| adv.机能上地，官能地 | |
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88
alluded
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| 提及，暗指( allude的过去式和过去分词 ) | |
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89
allude
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| v.提及，暗指 | |
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90
reptiles
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|
| n.爬行动物，爬虫( reptile的名词复数 ) | |
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91
incipient
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|
| adj.起初的，发端的，初期的 | |
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92
devoured
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|
| 吞没( devour的过去式和过去分词 )； 耗尽； 津津有味地看； 狼吞虎咽地吃光 | |
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93
decided
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|
| adj.决定了的，坚决的；明显的，明确的 | |
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94
simultaneously
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| adv.同时发生地，同时进行地 | |
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95
mere
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| adj.纯粹的；仅仅，只不过 | |
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96
tyrant
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| n.暴君，专制的君主，残暴的人 | |
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97
hovering
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| 鸟( hover的现在分词 )； 靠近（某事物）； （人）徘徊； 犹豫 | |
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98
proceeding
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|
| n.行动，进行，(pl.)会议录，学报 | |
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99
standing
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| n.持续，地位；adj.永久的，不动的，直立的，不流动的 | |
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100
margin
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| n.页边空白；差额；余地，余裕；边，边缘 | |
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101
yew
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| n.紫杉属树木 | |
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102
catching
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|
| adj.易传染的，有魅力的，迷人的，接住 | |
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103
monstrous
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|
| adj.巨大的；恐怖的；可耻的，丢脸的 | |
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104
anomalous
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|
| adj.反常的；不规则的 | |
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105
considerably
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|
| adv.极大地；相当大地；在很大程度上 | |
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106
organisation
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| n.组织，安排，团体，有机休 | |
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107
unwillingly
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| adv.不情愿地 | |
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108
subsists
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|
| v.（靠很少的钱或食物）维持生活，生存下去( subsist的第三人称单数 ) | |
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109
quail
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|
| n.鹌鹑；vi.畏惧，颤抖 | |
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110
dignified
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| a.可敬的，高贵的 | |
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111
spherical
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|
| adj.球形的；球面的 | |
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112
chromatic
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|
| adj.色彩的，颜色的 | |
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113
aberration
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| n.离开正路，脱离常规，色差 | |
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114
vibrations
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|
| n.摆动( vibration的名词复数 )；震动；感受；（偏离平衡位置的）一次性往复振动 | |
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115
collateral
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|
| adj.平行的；旁系的；n.担保品 | |
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116
lateral
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| adj.侧面的，旁边的 | |
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117
stratum
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| n.地层，社会阶层 | |
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118
pigment
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| n.天然色素，干粉颜料 | |
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119
mechanism
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| n.机械装置；机构，结构 | |
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120
crustaceans
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| n.甲壳纲动物（如蟹、龙虾）( crustacean的名词复数 ) | |
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121
facets
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|
| n.（宝石或首饰的）小平面( facet的名词复数 )；（事物的）面；方面 | |
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122
swelling
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| n.肿胀 | |
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123
transparent
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|
| adj.明显的，无疑的；透明的 | |
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124
cones
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|
| n.（人眼）圆锥细胞；圆锥体( cone的名词复数 )；球果；圆锥形东西；（盛冰淇淋的）锥形蛋卷筒 | |
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125
briefly
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| adv.简单地，简短地 | |
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126
possessed
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| adj.疯狂的；拥有的，占有的 | |
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127
treatise
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| n.专著；(专题)论文 | |
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128
inexplicable
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|
| adj.无法解释的，难理解的 | |
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129
hesitation
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| n.犹豫，踌躇 | |
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130
analogous
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| adj.相似的；类似的 | |
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131
presumptuous
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| adj.胆大妄为的，放肆的，冒昧的，冒失的 | |
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132
density
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| n.密集，密度，浓度 | |
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133
densities
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| 密集( density的名词复数 )； 稠密； 密度（固体、液体或气体单位体积的质量）； 密度（磁盘存贮数据的可用空间） | |
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134
alteration
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| n.变更，改变；蚀变 | |
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135
varied
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| adj.多样的，多变化的 | |
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136
alterations
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| n.改动( alteration的名词复数 )；更改；变化；改变 | |
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137
infinitely
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| adv.无限地，无穷地 | |
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138
alimentary
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|
| adj.饮食的，营养的 | |
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139
hydra
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| n.水螅；难于根除的祸患 | |
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140
exterior
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|
| adj.外部的，外在的；表面的 | |
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141
adder
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| n.蝰蛇；小毒蛇 | |
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142
vascular
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|
| adj.血管的，脉管的 | |
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143
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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144
respiration
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|
| n.呼吸作用；一次呼吸；植物光合作用 | |
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145
complement
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| n.补足物，船上的定员；补语；vt.补充，补足 | |
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146
physiologists
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|
| n.生理学者( physiologist的名词复数 )；生理学( physiology的名词复数 )；生理机能 | |
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147
slits
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|
| n.狭长的口子，裂缝( slit的名词复数 )v.切开，撕开( slit的第三人称单数 )；在…上开狭长口子 | |
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148
arteries
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|
| n.动脉( artery的名词复数 )；干线，要道 | |
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149
embryo
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| n.胚胎，萌芽的事物 | |
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150
utterly
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| adv.完全地，绝对地 | |
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151
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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152
naturalist
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|
| n.博物学家(尤指直接观察动植物者) | |
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153
dorsal
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| adj.背部的，背脊的 | |
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154
conversion
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| n.转化，转换，转变 | |
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155
secretion
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| n.分泌 | |
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156
obliteration
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|
| n.涂去，删除；管腔闭合 | |
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157
adhesive
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|
| n.粘合剂；adj.可粘着的，粘性的 | |
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158
glands
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| n.腺( gland的名词复数 ) | |
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159
gland
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| n.腺体，(机)密封压盖，填料盖 | |
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160
undoubtedly
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|
| adv.确实地，无疑地 | |
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161
wondrous
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|
| adj.令人惊奇的，奇妙的；adv.惊人地；异乎寻常地；令人惊叹地 | |
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162
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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163
progenitor
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| n.祖先，先驱 | |
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164
luminous
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|
| adj.发光的，发亮的；光明的；明白易懂的；有启发的 | |
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165
pollen
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|
| n.[植]花粉 | |
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166
apparently
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| adv.显然地；表面上，似乎 | |
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167
conjecture
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| n./v.推测，猜测 | |
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168
prodigal
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|
| adj.浪费的，挥霍的，放荡的 | |
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169
deviation
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| n.背离，偏离；偏差，偏向；离题 | |
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170
incessantly
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| ad.不停地 | |
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171
harassed
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| adj. 疲倦的，厌烦的 动词harass的过去式和过去分词 | |
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dearth
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| n.缺乏，粮食不足，饥谨 | |
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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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locomotion
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| n.运动，移动 | |
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correlation
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| n.相互关系，相关，关连 | |
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exquisitely
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| adv.精致地；强烈地；剧烈地；异常地 | |
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putrid
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| adj.腐臭的；有毒的；已腐烂的；卑劣的 | |
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putridity
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| n.腐败 | |
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skulls
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| 颅骨( skull的名词复数 )； 脑袋； 脑子； 脑瓜 | |
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parturition
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| n.生产，分娩 | |
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domesticated
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| adj.喜欢家庭生活的；（指动物）被驯养了的v.驯化( domesticate的过去式和过去分词 ) | |
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civilized
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| a.有教养的，文雅的 | |
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affected
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| adj.不自然的，假装的 | |
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laborious
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| adj.吃力的，努力的，不流畅 | |
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185
savages
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| 未开化的人，野蛮人( savage的名词复数 ) | |
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186
savage
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| adj.野蛮的；凶恶的，残暴的；n.未开化的人 | |
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copious
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| adj.丰富的，大量的 | |
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188
frivolous
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| adj.轻薄的；轻率的 | |
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utilitarian
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| adj.实用的，功利的 | |
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doctrine
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| n.教义；主义；学说 | |
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entailed
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| 使…成为必要( entail的过去式和过去分词 )； 需要； 限定继承； 使必需 | |
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flipper
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| n. 鳍状肢，潜水用橡皮制鳍状肢 | |
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progenitors
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| n.祖先( progenitor的名词复数 )；先驱；前辈；原本 | |
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fang
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| n.尖牙，犬牙 | |
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annihilate
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| v.使无效；毁灭；取消 | |
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rattle
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| v.飞奔，碰响；激怒；n.碰撞声；拨浪鼓 | |
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doomed
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| 命定的 | |
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advantageous
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| adj.有利的；有帮助的 | |
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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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myriads
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| n.无数，极大数量( myriad的名词复数 ) | |
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attained
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| （通常经过努力）实现( attain的过去式和过去分词 )； 达到； 获得； 达到（某年龄、水平、状况） | |
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wasp
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| n.黄蜂，蚂蜂 | |
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withdrawn
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| vt.收回；使退出；vi.撤退，退出 | |
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inevitably
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| adv.不可避免地；必然发生地 | |
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galls
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| v.使…擦痛( gall的第三人称单数 )；擦伤；烦扰；侮辱 | |
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intensified
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| v.（使）增强， （使）加剧( intensify的过去式和过去分词 ) | |
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slaughtered
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| v.屠杀，杀戮，屠宰( slaughter的过去式和过去分词 ) | |
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industrious
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| adj.勤劳的，刻苦的，奋发的 | |
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instinctive
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| adj.(出于)本能的；直觉的；(出于)天性的 | |
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hatred
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| n.憎恶，憎恨，仇恨 | |
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maternal
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| adj.母亲的，母亲般的，母系的，母方的 | |
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dense
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| a.密集的，稠密的，浓密的；密度大的 | |
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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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supplanting
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| 把…排挤掉，取代( supplant的现在分词 ) | |
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exterminating
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| v.消灭，根绝( exterminate的现在分词 ) | |
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complexity
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| n.复杂(性)，复杂的事物 | |
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217
faculties
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| n.能力( faculty的名词复数 )；全体教职员；技巧；院 | |
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inorganic
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| adj.无生物的；无机的 | |
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