Bears on natural selection. The term used in a wide sense. Geometrical powers of increase. Rapid increase of naturalised animals and plants. Nature of the checks to increase. Competition universal. Effects of climate. Protection from the number of individuals. Complex relations of all animals and plants throughout nature. Struggle for life most severe between individuals and varieties of the same species; often severe between species of the same genus. The relation of organism to organism the most important of all relations.
Before entering on the subject of this chapter, I must make a few preliminary remarks, to show how the struggle for existence bears on Natural Selection. It has been seen in the last chapter that amongst organic beings in a state of nature there is some individual variability; indeed I am not aware that this has ever been disputed. It is immaterial for us whether a multitude of doubtful forms be called species or sub-species or varieties; what rank, for instance, the two or three hundred doubtful forms of British plants are entitled to hold, if the existence of any well-marked varieties be admitted. But the mere1 existence of individual variability and of some few well-marked varieties, though necessary as the foundation for the work, helps us but little in understanding how species arise in nature. How have all those exquisite3 adaptations of one part of the organisation4 to another part, and to the conditions of life, and of one distinct organic being to another being, been perfected? We see these beautiful co-adaptations most plainly in the woodpecker and missletoe; and only a little less plainly in the humblest parasite5 which clings to the hairs of a quadruped or feathers of a bird; in the structure of the beetle6 which dives through the water; in the plumed7 seed which is wafted8 by the gentlest breeze; in short, we see beautiful adaptations everywhere and in every part of the organic world.
Again, it may be asked, how is it that varieties, which I have called incipient9 species, become ultimately converted into good and distinct species, which in most cases obviously differ from each other far more than do the varieties of the same species? How do those groups of species, which constitute what are called distinct genera, and which differ from each other more than do the species of the same genus, arise? All these results, as we shall more fully10 see in the next chapter, follow inevitably11 from the struggle for life. Owing to this struggle for life, any variation, however slight and from whatever cause proceeding12, if it be in any degree profitable to an individual of any species, in its infinitely13 complex relations to other organic beings and to external nature, will tend to the preservation14 of that individual, and will generally be inherited by its offspring. The offspring, also, will thus have a better chance of surviving, for, of the many individuals of any species which are periodically born, but a small number can survive. I have called this principle, by which each slight variation, if useful, is preserved, by the term of Natural Selection, in order to mark its relation to man's power of selection. We have seen that man by selection can certainly produce great results, and can adapt organic beings to his own uses, through the accumulation of slight but useful variations, given to him by the hand of Nature. But Natural Selection, as we shall hereafter see, is a power incessantly16 ready for action, and is as immeasurably superior to man's feeble efforts, as the works of Nature are to those of Art.
We will now discuss in a little more detail the struggle for existence. In my future work this subject shall be treated, as it well deserves, at much greater length. The elder De Candolle and Lyell have largely and philosophically17 shown that all organic beings are exposed to severe competition. In regard to plants, no one has treated this subject with more spirit and ability than W. Herbert, Dean of Manchester, evidently the result of his great horticultural knowledge. Nothing is easier than to admit in words the truth of the universal struggle for life, or more difficult--at least I have found it so--than constantly to bear this conclusion in mind. Yet unless it be thoroughly18 engrained in the mind, I am convinced that the whole economy of nature, with every fact on distribution, rarity, abundance, extinction19, and variation, will be dimly seen or quite misunderstood. We behold20 the face of nature bright with gladness, we often see superabundance of food; we do not see, or we forget, that the birds which are idly singing round us mostly live on insects or seeds, and are thus constantly destroying life; or we forget how largely these songsters, or their eggs, or their nestlings, are destroyed by birds and beasts of prey21; we do not always bear in mind, that though food may be now superabundant, it is not so at all seasons of each recurring22 year.
I should premise23 that I use the term Struggle for Existence in a large and metaphorical24 sense, including dependence25 of one being on another, and including (which is more important) not only the life of the individual, but success in leaving progeny26. Two canine27 animals in a time of dearth28, may be truly said to struggle with each other which shall get food and live. But a plant on the edge of a desert is said to struggle for life against the drought, though more properly it should be said to be dependent on the moisture. A plant which annually29 produces a thousand seeds, of which on an average only one comes to maturity30, may be more truly said to struggle with the plants of the same and other kinds which already clothe the ground. The missletoe is dependent on the apple and a few other trees, but can only in a far-fetched sense be said to struggle with these trees, for if too many of these parasites31 grow on the same tree, it will languish32 and die. But several seedling33 missletoes, growing close together on the same branch, may more truly be said to struggle with each other. As the missletoe is disseminated35 by birds, its existence depends on birds; and it may metaphorically36 be said to struggle with other fruit-bearing plants, in order to tempt37 birds to devour38 and thus disseminate34 its seeds rather than those of other plants. In these several senses, which pass into each other, I use for convenience sake the general term of struggle for existence.
A struggle for existence inevitably follows from the high rate at which all organic beings tend to increase. Every being, which during its natural lifetime produces several eggs or seeds, must suffer destruction during some period of its life, and during some season or occasional year, otherwise, on the principle of geometrical increase, its numbers would quickly become so inordinately39 great that no country could support the product. Hence, as more individuals are produced than can possibly survive, there must in every case be a struggle for existence, either one individual with another of the same species, or with the individuals of distinct species, or with the physical conditions of life. It is the doctrine40 of Malthus applied41 with manifold force to the whole animal and vegetable kingdoms; for in this case there can be no artificial increase of food, and no prudential restraint from marriage. Although some species may be now increasing, more or less rapidly, in numbers, all cannot do so, for the world would not hold them.
There is no exception to the rule that every organic being naturally increases at so high a rate, that if not destroyed, the earth would soon be covered by the progeny of a single pair. Even slow-breeding man has doubled in twenty-five years, and at this rate, in a few thousand years, there would literally42 not be standing2 room for his progeny. Linnaeus has calculated that if an annual plant produced only two seeds--and there is no plant so unproductive as this--and their seedlings43 next year produced two, and so on, then in twenty years there would be a million plants. The elephant is reckoned to be the slowest breeder of all known animals, and I have taken some pains to estimate its probable minimum rate of natural increase: it will be under the mark to assume that it breeds when thirty years old, and goes on breeding till ninety years old, bringing forth44 three pair of young in this interval45; if this be so, at the end of the fifth century there would be alive fifteen million elephants, descended46 from the first pair.
But we have better evidence on this subject than mere theoretical calculations, namely, the numerous recorded cases of the astonishingly rapid increase of various animals in a state of nature, when circumstances have been favourable47 to them during two or three following seasons. Still more striking is the evidence from our domestic animals of many kinds which have run wild in several parts of the world: if the statements of the rate of increase of slow-breeding cattle and horses in South America, and latterly in Australia, had not been well authenticated48, they would have been quite incredible. So it is with plants: cases could be given of introduced plants which have become common throughout whole islands in a period of less than ten years. Several of the plants now most numerous over the wide plains of La Plata, clothing square leagues of surface almost to the exclusion50 of all other plants, have been introduced from Europe; and there are plants which now range in India, as I hear from Dr. Falconer, from Cape51 Comorin to the Himalaya, which have been imported from America since its discovery. In such cases, and endless instances could be given, no one supposes that the fertility of these animals or plants has been suddenly and temporarily increased in any sensible degree. The obvious explanation is that the conditions of life have been very favourable, and that there has consequently been less destruction of the old and young, and that nearly all the young have been enabled to breed. In such cases the geometrical ratio of increase, the result of which never fails to be surprising, simply explains the extraordinarily52 rapid increase and wide diffusion53 of naturalised productions in their new homes.
In a state of nature almost every plant produces seed, and amongst animals there are very few which do not annually pair. Hence we may confidently assert, that all plants and animals are tending to increase at a geometrical ratio, that all would most rapidly stock every station in which they could any how exist, and that the geometrical tendency to increase must be checked by destruction at some period of life. Our familiarity with the larger domestic animals tends, I think, to mislead us: we see no great destruction falling on them, and we forget that thousands are annually slaughtered54 for food, and that in a state of nature an equal number would have somehow to be disposed of.
The only difference between organisms which annually produce eggs or seeds by the thousand, and those which produce extremely few, is, that the slow-breeders would require a few more years to people, under favourable conditions, a whole district, let it be ever so large. The condor55 lays a couple of eggs and the ostrich56 a score, and yet in the same country the condor may be the more numerous of the two: the Fulmar petrel lays but one egg, yet it is believed to be the most numerous bird in the world. One fly deposits hundreds of eggs, and another, like the hippobosca, a single one; but this difference does not determine how many individuals of the two species can be supported in a district. A large number of eggs is of some importance to those species, which depend on a rapidly fluctuating amount of food, for it allows them rapidly to increase in number. But the real importance of a large number of eggs or seeds is to make up for much destruction at some period of life; and this period in the great majority of cases is an early one. If an animal can in any way protect its own eggs or young, a small number may be produced, and yet the average stock be fully kept up; but if many eggs or young are destroyed, many must be produced, or the species will become extinct. It would suffice to keep up the full number of a tree, which lived on an average for a thousand years, if a single seed were produced once in a thousand years, supposing that this seed were never destroyed, and could be ensured to germinate57 in a fitting place. So that in all cases, the average number of any animal or plant depends only indirectly58 on the number of its eggs or seeds.
In looking at Nature, it is most necessary to keep the foregoing considerations always in mind--never to forget that every single organic being around us may be said to be striving to the utmost to increase in numbers; that each lives by a struggle at some period of its life; that heavy destruction inevitably falls either on the young or old, during each generation or at recurrent intervals59. Lighten any check, mitigate60 the destruction ever so little, and the number of the species will almost instantaneously increase to any amount. The face of Nature may be compared to a yielding surface, with ten thousand sharp wedges packed close together and driven inwards by incessant15 blows, sometimes one wedge being struck, and then another with greater force.
What checks the natural tendency of each species to increase in number is most obscure. Look at the most vigorous species; by as much as it swarms62 in numbers, by so much will its tendency to increase be still further increased. We know not exactly what the checks are in even one single instance. Nor will this surprise any one who reflects how ignorant we are on this head, even in regard to mankind, so incomparably better known than any other animal. This subject has been ably treated by several authors, and I shall, in my future work, discuss some of the checks at considerable length, more especially in regard to the feral animals of South America. Here I will make only a few remarks, just to recall to the reader's mind some of the chief points. Eggs or very young animals seem generally to suffer most, but this is not invariably the case. With plants there is a vast destruction of seeds, but, from some observations which I have made, I believe that it is the seedlings which suffer most from germinating63 in ground already thickly stocked with other plants. Seedlings, also, are destroyed in vast numbers by various enemies; for instance, on a piece of ground three feet long and two wide, dug and cleared, and where there could be no choking from other plants, I marked all the seedlings of our native weeds as they came up, and out of the 357 no less than 295 were destroyed, chiefly by slugs and insects. If turf which has long been mown, and the case would be the same with turf closely browsed64 by quadrupeds, be let to grow, the more vigorous plants gradually kill the less vigorous, though fully grown, plants: thus out of twenty species growing on a little plot of turf (three feet by four) nine species perished from the other species being allowed to grow up freely.
The amount of food for each species of course gives the extreme limit to which each can increase; but very frequently it is not the obtaining food, but the serving as prey to other animals, which determines the average numbers of a species. Thus, there seems to be little doubt that the stock of partridges, grouse65, and hares on any large estate depends chiefly on the destruction of vermin. If not one head of game were shot during the next twenty years in England, and, at the same time, if no vermin were destroyed, there would, in all probability, be less game than at present, although hundreds of thousands of game animals are now annually killed. On the other hand, in some cases, as with the elephant and rhinoceros66, none are destroyed by beasts of prey: even the tiger in India most rarely dares to attack a young elephant protected by its dam.
Climate plays an important part in determining the average numbers of a species, and periodical seasons of extreme cold or drought, I believe to be the most effective of all checks. I estimated that the winter of 1854-55 destroyed four-fifths of the birds in my own grounds; and this is a tremendous destruction, when we remember that ten per cent. is an extraordinarily severe mortality from epidemics67 with man. The action of climate seems at first sight to be quite independent of the struggle for existence; but in so far as climate chiefly acts in reducing food, it brings on the most severe struggle between the individuals, whether of the same or of distinct species, which subsist68 on the same kind of food. Even when climate, for instance extreme cold, acts directly, it will be the least vigorous, or those which have got least food through the advancing winter, which will suffer most. When we travel from south to north, or from a damp region to a dry, we invariably see some species gradually getting rarer and rarer, and finally disappearing; and the change of climate being conspicuous69, we are tempted70 to attribute the whole effect to its direct action. But this is a very false view: we forget that each species, even where it most abounds71, is constantly suffering enormous destruction at some period of its life, from enemies or from competitors for the same place and food; and if these enemies or competitors be in the least degree favoured by any slight change of climate, they will increase in numbers, and, as each area is already fully stocked with inhabitants, the other species will decrease. When we travel southward and see a species decreasing in numbers, we may feel sure that the cause lies quite as much in other species being favoured, as in this one being hurt. So it is when we travel northward72, but in a somewhat lesser73 degree, for the number of species of all kinds, and therefore of competitors, decreases northwards; hence in going northward, or in ascending74 a mountain, we far oftener meet with stunted75 forms, due to the DIRECTLY injurious action of climate, than we do in proceeding southwards or in descending76 a mountain. When we reach the Arctic regions, or snow-capped summits, or absolute deserts, the struggle for life is almost exclusively with the elements.
That climate acts in main part indirectly by favouring other species, we may clearly see in the prodigious77 number of plants in our gardens which can perfectly78 well endure our climate, but which never become naturalised, for they cannot compete with our native plants, nor resist destruction by our native animals.
When a species, owing to highly favourable circumstances, increases inordinately in numbers in a small tract79, epidemics--at least, this seems generally to occur with our game animals--often ensue: and here we have a limiting check independent of the struggle for life. But even some of these so-called epidemics appear to be due to parasitic80 worms, which have from some cause, possibly in part through facility of diffusion amongst the crowded animals, been disproportionably favoured: and here comes in a sort of struggle between the parasite and its prey.
On the other hand, in many cases, a large stock of individuals of the same species, relatively81 to the numbers of its enemies, is absolutely necessary for its preservation. Thus we can easily raise plenty of corn and rape-seed, etc., in our fields, because the seeds are in great excess compared with the number of birds which feed on them; nor can the birds, though having a superabundance of food at this one season, increase in number proportionally to the supply of seed, as their numbers are checked during winter: but any one who has tried, knows how troublesome it is to get seed from a few wheat or other such plants in a garden; I have in this case lost every single seed. This view of the necessity of a large stock of the same species for its preservation, explains, I believe, some singular facts in nature, such as that of very rare plants being sometimes extremely abundant in the few spots where they do occur; and that of some social plants being social, that is, abounding82 in individuals, even on the extreme confines of their range. For in such cases, we may believe, that a plant could exist only where the conditions of its life were so favourable that many could exist together, and thus save each other from utter destruction. I should add that the good effects of frequent intercrossing, and the ill effects of close interbreeding, probably come into play in some of these cases; but on this intricate subject I will not here enlarge.
Many cases are on record showing how complex and unexpected are the checks and relations between organic beings, which have to struggle together in the same country. I will give only a single instance, which, though a simple one, has interested me. In Staffordshire, on the estate of a relation where I had ample means of investigation83, there was a large and extremely barren heath, which had never been touched by the hand of man; but several hundred acres of exactly the same nature had been enclosed twenty-five years previously84 and planted with Scotch85 fir. The change in the native vegetation of the planted part of the heath was most remarkable86, more than is generally seen in passing from one quite different soil to another: not only the proportional numbers of the heath-plants were wholly changed, but twelve species of plants (not counting grasses and carices) flourished in the plantations87, which could not be found on the heath. The effect on the insects must have been still greater, for six insectivorous birds were very common in the plantations, which were not to be seen on the heath; and the heath was frequented by two or three distinct insectivorous birds. Here we see how potent88 has been the effect of the introduction of a single tree, nothing whatever else having been done, with the exception that the land had been enclosed, so that cattle could not enter. But how important an element enclosure is, I plainly saw near Farnham, in Surrey. Here there are extensive heaths, with a few clumps89 of old Scotch firs on the distant hill-tops: within the last ten years large spaces have been enclosed, and self-sown firs are now springing up in multitudes, so close together that all cannot live.
When I ascertained90 that these young trees had not been sown or planted, I was so much surprised at their numbers that I went to several points of view, whence I could examine hundreds of acres of the unenclosed heath, and literally I could not see a single Scotch fir, except the old planted clumps. But on looking closely between the stems of the heath, I found a multitude of seedlings and little trees, which had been perpetually browsed down by the cattle. In one square yard, at a point some hundred yards distant from one of the old clumps, I counted thirty-two little trees; and one of them, judging from the rings of growth, had during twenty-six years tried to raise its head above the stems of the heath, and had failed. No wonder that, as soon as the land was enclosed, it became thickly clothed with vigorously growing young firs. Yet the heath was so extremely barren and so extensive that no one would ever have imagined that cattle would have so closely and effectually searched it for food.
Here we see that cattle absolutely determine the existence of the Scotch fir; but in several parts of the world insects determine the existence of cattle. Perhaps Paraguay offers the most curious instance of this; for here neither cattle nor horses nor dogs have ever run wild, though they swarm61 southward and northward in a feral state; and Azara and Rengger have shown that this is caused by the greater number in Paraguay of a certain fly, which lays its eggs in the navels of these animals when first born. The increase of these flies, numerous as they are, must be habitually91 checked by some means, probably by birds. Hence, if certain insectivorous birds (whose numbers are probably regulated by hawks92 or beasts of prey) were to increase in Paraguay, the flies would decrease--then cattle and horses would become feral, and this would certainly greatly alter (as indeed I have observed in parts of South America) the vegetation: this again would largely affect the insects; and this, as we just have seen in Staffordshire, the insectivorous birds, and so onwards in ever-increasing circles of complexity93. We began this series by insectivorous birds, and we have ended with them. Not that in nature the relations can ever be as simple as this. Battle within battle must ever be recurring with varying success; and yet in the long-run the forces are so nicely balanced, that the face of nature remains94 uniform for long periods of time, though assuredly the merest trifle would often give the victory to one organic being over another. Nevertheless so profound is our ignorance, and so high our presumption95, that we marvel96 when we hear of the extinction of an organic being; and as we do not see the cause, we invoke97 cataclysms98 to desolate99 the world, or invent laws on the duration of the forms of life!
I am tempted to give one more instance showing how plants and animals, most remote in the scale of nature, are bound together by a web of complex relations. I shall hereafter have occasion to show that the exotic Lobelia fulgens, in this part of England, is never visited by insects, and consequently, from its peculiar100 structure, never can set a seed. Many of our orchidaceous plants absolutely require the visits of moths101 to remove their pollen-masses and thus to fertilise them. I have, also, reason to believe that humble-bees are indispensable to the fertilisation of the heartsease (Viola tricolor), for other bees do not visit this flower. From experiments which I have tried, I have found that the visits of bees, if not indispensable, are at least highly beneficial to the fertilisation of our clovers; but humble-bees alone visit the common red clover (Trifolium pratense), as other bees cannot reach the nectar. Hence I have very little doubt, that if the whole genus of humble-bees became extinct or very rare in England, the heartsease and red clover would become very rare, or wholly disappear. The number of humble-bees in any district depends in a great degree on the number of field-mice, which destroy their combs and nests; and Mr. H. Newman, who has long attended to the habits of humble-bees, believes that "more than two thirds of them are thus destroyed all over England." Now the number of mice is largely dependent, as every one knows, on the number of cats; and Mr. Newman says, "Near villages and small towns I have found the nests of humble-bees more numerous than elsewhere, which I attribute to the number of cats that destroy the mice." Hence it is quite credible49 that the presence of a feline102 animal in large numbers in a district might determine, through the intervention103 first of mice and then of bees, the frequency of certain flowers in that district!
In the case of every species, many different checks, acting104 at different periods of life, and during different seasons or years, probably come into play; some one check or some few being generally the most potent, but all concurring105 in determining the average number or even the existence of the species. In some cases it can be shown that widely-different checks act on the same species in different districts. When we look at the plants and bushes clothing an entangled106 bank, we are tempted to attribute their proportional numbers and kinds to what we call chance. But how false a view is this! Every one has heard that when an American forest is cut down, a very different vegetation springs up; but it has been observed that the trees now growing on the ancient Indian mounds107, in the Southern United States, display the same beautiful diversity and proportion of kinds as in the surrounding virgin108 forests. What a struggle between the several kinds of trees must here have gone on during long centuries, each annually scattering109 its seeds by the thousand; what war between insect and insect--between insects, snails110, and other animals with birds and beasts of prey--all striving to increase, and all feeding on each other or on the trees or their seeds and seedlings, or on the other plants which first clothed the ground and thus checked the growth of the trees! Throw up a handful of feathers, and all must fall to the ground according to definite laws; but how simple is this problem compared to the action and reaction of the innumerable plants and animals which have determined111, in the course of centuries, the proportional numbers and kinds of trees now growing on the old Indian ruins!
The dependency of one organic being on another, as of a parasite on its prey, lies generally between beings remote in the scale of nature. This is often the case with those which may strictly112 be said to struggle with each other for existence, as in the case of locusts113 and grass-feeding quadrupeds. But the struggle almost invariably will be most severe between the individuals of the same species, for they frequent the same districts, require the same food, and are exposed to the same dangers. In the case of varieties of the same species, the struggle will generally be almost equally severe, and we sometimes see the contest soon decided114: for instance, if several varieties of wheat be sown together, and the mixed seed be resown, some of the varieties which best suit the soil or climate, or are naturally the most fertile, will beat the others and so yield more seed, and will consequently in a few years quite supplant115 the other varieties. To keep up a mixed stock of even such extremely close varieties as the variously coloured sweet-peas, they must be each year harvested separately, and the seed then mixed in due proportion, otherwise the weaker kinds will steadily116 decrease in numbers and disappear. So again with the varieties of sheep: it has been asserted that certain mountain-varieties will starve out other mountain-varieties, so that they cannot be kept together. The same result has followed from keeping together different varieties of the medicinal leech117. It may even be doubted whether the varieties of any one of our domestic plants or animals have so exactly the same strength, habits, and constitution, that the original proportions of a mixed stock could be kept up for half a dozen generations, if they were allowed to struggle together, like beings in a state of nature, and if the seed or young were not annually sorted.
As species of the same genus have usually, though by no means invariably, some similarity in habits and constitution, and always in structure, the struggle will generally be more severe between species of the same genus, when they come into competition with each other, than between species of distinct genera. We see this in the recent extension over parts of the United States of one species of swallow having caused the decrease of another species. The recent increase of the missel-thrush in parts of Scotland has caused the decrease of the song-thrush. How frequently we hear of one species of rat taking the place of another species under the most different climates! In Russia the small Asiatic cockroach118 has everywhere driven before it its great congener. One species of charlock will supplant another, and so in other cases. We can dimly see why the competition should be most severe between allied119 forms, which fill nearly the same place in the economy of nature; but probably in no one case could we precisely120 say why one species has been victorious121 over another in the great battle of life.
A corollary of the highest importance may be deduced from the foregoing remarks, namely, that the structure of every organic being is related, in the most essential yet often hidden manner, to that of all other organic beings, with which it comes into competition for food or residence, or from which it has to escape, or on which it preys122. This is obvious in the structure of the teeth and talons123 of the tiger; and in that of the legs and claws of the parasite which clings to the hair on the tiger's body. But in the beautifully plumed seed of the dandelion, and in the flattened124 and fringed legs of the water-beetle, the relation seems at first confined to the elements of air and water. Yet the advantage of plumed seeds no doubt stands in the closest relation to the land being already thickly clothed by other plants; so that the seeds may be widely distributed and fall on unoccupied ground. In the water-beetle, the structure of its legs, so well adapted for diving, allows it to compete with other aquatic125 insects, to hunt for its own prey, and to escape serving as prey to other animals.
The store of nutriment laid up within the seeds of many plants seems at first sight to have no sort of relation to other plants. But from the strong growth of young plants produced from such seeds (as peas and beans), when sown in the midst of long grass, I suspect that the chief use of the nutriment in the seed is to favour the growth of the young seedling, whilst struggling with other plants growing vigorously all around.
Look at a plant in the midst of its range, why does it not double or quadruple its numbers? We know that it can perfectly well withstand a little more heat or cold, dampness or dryness, for elsewhere it ranges into slightly hotter or colder, damper or drier districts. In this case we can clearly see that if we wished in imagination to give the plant the power of increasing in number, we should have to give it some advantage over its competitors, or over the animals which preyed126 on it. On the confines of its geographical127 range, a change of constitution with respect to climate would clearly be an advantage to our plant; but we have reason to believe that only a few plants or animals range so far, that they are destroyed by the rigour of the climate alone. Not until we reach the extreme confines of life, in the arctic regions or on the borders of an utter desert, will competition cease. The land may be extremely cold or dry, yet there will be competition between some few species, or between the individuals of the same species, for the warmest or dampest spots.
Hence, also, we can see that when a plant or animal is placed in a new country amongst new competitors, though the climate may be exactly the same as in its former home, yet the conditions of its life will generally be changed in an essential manner. If we wished to increase its average numbers in its new home, we should have to modify it in a different way to what we should have done in its native country; for we should have to give it some advantage over a different set of competitors or enemies.
It is good thus to try in our imagination to give any form some advantage over another. Probably in no single instance should we know what to do, so as to succeed. It will convince us of our ignorance on the mutual128 relations of all organic beings; a conviction as necessary, as it seems to be difficult to acquire. All that we can do, is to keep steadily in mind that each organic being is striving to increase at a geometrical ratio; that each at some period of its life, during some season of the year, during each generation or at intervals, has to struggle for life, and to suffer great destruction. When we reflect on this struggle, we may console ourselves with the full belief, that the war of nature is not incessant, that no fear is felt, that death is generally prompt, and that the vigorous, the healthy, and the happy survive and multiply.
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mere
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| adj.纯粹的；仅仅，只不过 | |
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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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exquisite
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| adj.精美的；敏锐的；剧烈的，感觉强烈的 | |
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organisation
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| n.组织，安排，团体，有机休 | |
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parasite
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| n.寄生虫；寄生菌；食客 | |
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beetle
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| n.甲虫，近视眼的人 | |
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plumed
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| 饰有羽毛的 | |
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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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incipient
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| adj.起初的，发端的，初期的 | |
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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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inevitably
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| adv.不可避免地；必然发生地 | |
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12
proceeding
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| n.行动，进行，(pl.)会议录，学报 | |
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13
infinitely
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| adv.无限地，无穷地 | |
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14
preservation
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| n.保护，维护，保存，保留，保持 | |
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15
incessant
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| adj.不停的，连续的 | |
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incessantly
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| ad.不停地 | |
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17
philosophically
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| adv.哲学上；富有哲理性地；贤明地；冷静地 | |
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18
thoroughly
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| adv.完全地，彻底地，十足地 | |
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19
extinction
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| n.熄灭，消亡，消灭，灭绝，绝种 | |
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20
behold
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| v.看，注视，看到 | |
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21
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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22
recurring
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| adj.往复的，再次发生的 | |
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23
premise
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| n.前提；v.提论，预述 | |
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24
metaphorical
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| a.隐喻的，比喻的 | |
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25
dependence
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| n.依靠，依赖；信任，信赖；隶属 | |
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26
progeny
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| n.后代，子孙；结果 | |
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27
canine
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| adj.犬的，犬科的 | |
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28
dearth
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| n.缺乏，粮食不足，饥谨 | |
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29
annually
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| adv.一年一次，每年 | |
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30
maturity
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| n.成熟；完成；(支票、债券等)到期 | |
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31
parasites
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| 寄生物( parasite的名词复数 )； 靠他人为生的人； 诸虫 | |
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32
languish
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| vi.变得衰弱无力，失去活力，（植物等）凋萎 | |
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33
seedling
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| n.秧苗，树苗 | |
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34
disseminate
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| v.散布；传播 | |
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35
disseminated
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| 散布，传播( disseminate的过去式和过去分词 ) | |
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36
metaphorically
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| adv. 用比喻地 | |
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37
tempt
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| vt.引诱，勾引，吸引，引起…的兴趣 | |
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38
devour
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| v.吞没；贪婪地注视或谛听，贪读；使着迷 | |
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39
inordinately
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| adv.无度地，非常地 | |
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40
doctrine
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| n.教义；主义；学说 | |
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41
applied
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| adj.应用的；v.应用，适用 | |
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42
literally
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| adv.照字面意义，逐字地；确实 | |
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43
seedlings
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| n.刚出芽的幼苗( seedling的名词复数 ) | |
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44
forth
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| adv.向前；向外，往外 | |
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45
interval
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| n.间隔，间距；幕间休息，中场休息 | |
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46
descended
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| a.为...后裔的，出身于...的 | |
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47
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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48
authenticated
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| v.证明是真实的、可靠的或有效的( authenticate的过去式和过去分词 )；鉴定，使生效 | |
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49
credible
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| adj.可信任的，可靠的 | |
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50
exclusion
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| n.拒绝，排除，排斥，远足，远途旅行 | |
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51
cape
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| n.海角，岬；披肩，短披风 | |
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52
extraordinarily
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| adv.格外地；极端地 | |
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53
diffusion
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| n.流布；普及；散漫 | |
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54
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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55
condor
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| n.秃鹰；秃鹰金币 | |
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56
ostrich
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| n.鸵鸟 | |
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57
germinate
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| v.发芽；发生；发展 | |
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58
indirectly
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| adv.间接地，不直接了当地 | |
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59
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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60
mitigate
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| vt.（使）减轻，（使）缓和 | |
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61
swarm
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| n.(昆虫)等一大群；vi.成群飞舞；蜂拥而入 | |
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62
swarms
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| 蜂群，一大群( swarm的名词复数 ) | |
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63
germinating
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| n.& adj.发芽（的）v.（使）发芽( germinate的现在分词 ) | |
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64
browsed
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| v.吃草( browse的过去式和过去分词 )；随意翻阅；（在商店里）随便看看；（在计算机上）浏览信息 | |
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65
grouse
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| n.松鸡；v.牢骚，诉苦 | |
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66
rhinoceros
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| n.犀牛 | |
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67
epidemics
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| n．流行病 | |
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68
subsist
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| vi.生存，存在，供养 | |
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69
conspicuous
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| adj.明眼的，惹人注目的；炫耀的，摆阔气的 | |
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70
tempted
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| v.怂恿（某人）干不正当的事；冒…的险（tempt的过去分词） | |
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71
abounds
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| v.大量存在，充满，富于( abound的第三人称单数 ) | |
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72
northward
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| adv.向北；n.北方的地区 | |
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73
lesser
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| adj.次要的，较小的；adv.较小地，较少地 | |
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74
ascending
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| adj.上升的，向上的 | |
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75
stunted
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| adj.矮小的；发育迟缓的 | |
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76
descending
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| n. 下行 adj. 下降的 | |
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77
prodigious
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| adj.惊人的，奇妙的；异常的；巨大的；庞大的 | |
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78
perfectly
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| adv.完美地，无可非议地，彻底地 | |
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79
tract
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| n.传单，小册子，大片(土地或森林) | |
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80
parasitic
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| adj.寄生的 | |
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81
relatively
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| adv.比较...地，相对地 | |
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82
abounding
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| adj.丰富的，大量的v.大量存在，充满，富于( abound的现在分词 ) | |
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83
investigation
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| n.调查，调查研究 | |
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84
previously
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| adv.以前，先前（地） | |
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85
scotch
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| n.伤口，刻痕；苏格兰威士忌酒；v.粉碎，消灭，阻止；adj.苏格兰(人)的 | |
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86
remarkable
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| adj.显著的，异常的，非凡的，值得注意的 | |
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87
plantations
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| n.种植园，大农场( plantation的名词复数 ) | |
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88
potent
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| adj.强有力的，有权势的；有效力的 | |
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89
clumps
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| n.（树、灌木、植物等的）丛、簇( clump的名词复数 )；（土、泥等）团；块；笨重的脚步声v.（树、灌木、植物等的）丛、簇( clump的第三人称单数 )；（土、泥等）团；块；笨重的脚步声 | |
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90
ascertained
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| v.弄清，确定，查明( ascertain的过去式和过去分词 ) | |
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91
habitually
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| ad.习惯地，通常地 | |
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92
hawks
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| 鹰( hawk的名词复数 )； 鹰派人物，主战派人物 | |
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93
complexity
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| n.复杂(性)，复杂的事物 | |
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94
remains
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| n.剩余物，残留物；遗体，遗迹 | |
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95
presumption
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| n.推测，可能性，冒昧，放肆，[法律]推定 | |
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96
marvel
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| vi.(at)惊叹vt.感到惊异；n.令人惊异的事 | |
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97
invoke
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| v.求助于(神、法律)；恳求，乞求 | |
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98
cataclysms
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| n.（突然降临的）大灾难( cataclysm的名词复数 ) | |
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99
desolate
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| adj.荒凉的，荒芜的；孤独的，凄凉的；v.使荒芜，使孤寂 | |
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100
peculiar
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| adj.古怪的，异常的；特殊的，特有的 | |
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101
moths
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| n.蛾( moth的名词复数 ) | |
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102
feline
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| adj.猫科的 | |
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103
intervention
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| n.介入，干涉，干预 | |
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104
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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105
concurring
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| 同时发生的，并发的 | |
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106
entangled
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| adj.卷入的；陷入的；被缠住的；缠在一起的v.使某人（某物/自己）缠绕，纠缠于（某物中），使某人（自己）陷入（困难或复杂的环境中）( entangle的过去式和过去分词 ) | |
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107
mounds
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| 土堆，土丘( mound的名词复数 )； 一大堆 | |
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108
virgin
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| n.处女，未婚女子；adj.未经使用的；未经开发的 | |
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109
scattering
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| n.[物]散射；散乱，分散；在媒介质中的散播adj.散乱的；分散在不同范围的；广泛扩散的；（选票）数量分散的v.散射(scatter的ing形式)；散布；驱散 | |
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110
snails
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| n.蜗牛；迟钝的人；蜗牛( snail的名词复数 ) | |
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111
determined
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| adj.坚定的；有决心的 | |
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112
strictly
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| adv.严厉地，严格地；严密地 | |
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113
locusts
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| n.蝗虫( locust的名词复数 )；贪吃的人；破坏者；槐树 | |
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114
decided
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| adj.决定了的，坚决的；明显的，明确的 | |
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115
supplant
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| vt.排挤；取代 | |
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116
steadily
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| adv.稳定地；不变地；持续地 | |
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117
leech
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| n.水蛭，吸血鬼，榨取他人利益的人；vt.以水蛭吸血；vi.依附于别人 | |
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118
cockroach
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| n.蟑螂 | |
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119
allied
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| adj.协约国的；同盟国的 | |
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120
precisely
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| adv.恰好，正好，精确地，细致地 | |
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121
victorious
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| adj.胜利的，得胜的 | |
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122
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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123
talons
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| n.（尤指猛禽的）爪( talon的名词复数 )；（如爪般的）手指；爪状物；锁簧尖状突出部 | |
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124
flattened
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| [医]（水）平扁的，弄平的 | |
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125
aquatic
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| adj.水生的，水栖的 | |
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126
preyed
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| v.掠食( prey的过去式和过去分词 )；掠食；折磨；（人）靠欺诈为生 | |
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127
geographical
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| adj.地理的；地区(性)的 | |
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128
mutual
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| adj.相互的，彼此的；共同的，共有的 | |
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