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shall fall compared to that of the action and reaction
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 cockroach 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 allied forms, which fill nearly the same place in the economy of nature ; but probably in no one case could we precisely say why one species has been victorious 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 preys. This is obvious in the structure of the teeth and talons 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 flattened 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 aquatic 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, it may be suspected 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
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 preyed on it. On the confines of its geographical range, a change of constitution with respect to climate would clearly he 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 to do 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 mutual 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 in a geometrical ratio ; tha 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.
Natural Selection—its power compared with man's selection—its
power on characters of trifling importance-its power at all ages and on both sexes-Sexual Selection-On the generality of intercrosses between individuals of the same species-Circumstances favourable and unfavourable to Natural Selection, namely, intercrossing, isolation, number of individuals—Slow actionExtinction caused by Natural Selection-Divergence of Character, related to the diversity of inhabitants of any small area, and to naturalisation-Action of Natural Selection, through Divergence of Character and Extinction, on the descendants from a common parent-Explains the Grouping of all organic beings Advance in organisation-Low forms preserved-Objections
considered-Indefinite multiplication of species—Summary. How will the struggle for existence, discussed too briefly in the last chapter, act in regard to variation ? Can the principle of selection, which we have seen is so potent in the hands of man, apply in nature ? I think we shall see that it can act most effectually. Let it be borne in mind in what an endless number of strange peculiarities our domestic productions, and, in a lesser degree, those under nature, vary; and how strong the hereditary tendency is. Under domestication, it may be truly said that the whole organisation becomes in some degree plastic. But the variability, which we almost universally meet with in our domestic productions is not directly produced, as Hooker and Asa Gray have well remarked, by man ; he can neither originate varieties, nor prevent their occurrence; he can only preserve and accumulate such as do occur ; unintentionally he exposes organic beings to new and changing conditions of life, and variability ensues; but similar changes of conditions might and do occur under nature. Let it also be borne in mind how infinitely complex and close-fitting are the mutual relations of all organic beings to each other and to their physical conditions of life ; and quently what infinitely varied diversities of structure may be of use to each being under changing conditions of life. (Can it, then, be thought improbable, seeing
that variations useful to man have undoubtedly occurred, fenat other variations useful in some way to each being in the great and complex battle of life, should sometimes occur in the course of thousands of generations ? If such do occur, can we doubt (remembering that many more individuals are born than can possibly survive) that findividuals having any advantage, however slight, over others, would have the best chance of surviving and of procreating their kind? On the other hand," we may feel sure that any variation in the least degree injurious would be rigidly destroyed. This preservation of favourable variations and the rejection of injurious variations, I call Natural Selection Variations neither useful nor injurious would not be affected by natural selection, and would be left a fluctuating element, as perhaps we see in the species called polymorphic.
Several writers have misapprehended or objected to the term Natural Selection. Some have even imagined that natural selection induces variability, whereas it implies only the preservation of such variations as occur and are beneficiat to the being under its conditions of life. No one objects to agriculturists speaking of the potent effects of man's selection, and in this case the individual differences given by nature, which man for some objects sélects, must of necessity first occur. Others have objected that the term selection implies conscious choice in the animals which become modified; and it has even been urged that as plants have no volition, natural selection is not applicable to them ! In the literal sense of the word, no doubt, natural selection is a misnomer ; but who ever objected to chemists speaking of the elective affinities of the various elements ?—and yet an acid cannot strictly be said to elect the base with which it will in preference combine. It has been said that I speak of natural selection as an active power or Deity ; but who objects to an author speaking of the attraction of gravity as ruling the movements of the planets ? Every one knows what is meant and is implied by such metaphorical expressions; and they are almost necessary for brevity. So again it is difficult to avoid personifying the word Nature ; but I mean by Nature, only the aggregate action and product of many natural laws, and by laws the sequence of events as ascertained by us. With a little familiarity such superficial objections will be forgotten.