[36] Animals and Plants under Domestication, vol. ii. pp. 307-311.
CHAPTER V
NATURAL SELECTION BY VARIATION AND SURVIVAL OF THE FITTEST
Effect of struggle for existence under unchanged conditions—The effect under change of conditions—Divergence of character—In insects—In birds—In mammalia—Divergence leads to a maximum of life in each area—Closely allied species inhabit distinct areas—Adaptation to conditions at various periods of life—The continued existence of low forms of life—Extinction of low types among the higher animals—Circumstances favourable to the origin of new species—Probable origin of the dippers—The importance of isolation—On the advance of organisation by natural selection—Summary of the first five chapters.
In the preceding chapters we have accumulated a body of facts and arguments which will enable us now to deal with the very core of our subject—the formation of species by means of natural selection. We have seen how tremendous is the struggle for existence always going on in nature owing to the great powers of increase of all organisms; we have ascertained the fact of variability extending to every part and organ, each of which varies simultaneously and for the most part independently; and we have seen that this variability is both large in its amount in proportion to the size of each part, and usually affects a considerable proportion of the individuals in the large and dominant species. And, lastly, we have seen how similar variations, occurring in cultivated plants and domestic animals, are capable of being perpetuated and accumulated by artificial selection, till they have resulted in all the wonderful varieties of our fruits, flowers, and vegetables, our domestic animals and household pets, many of which differ from each other far more in external characters, habits, and instincts than do species in a state of nature. We have now to inquire whether there is any analogous process in nature, by which wild animals and plants can be permanently modified and new races or new species produced.
Effect of Struggle for Existence under Unchanged Conditions.
Let us first consider what will be the effect of the struggle for existence upon the animals and plants which we see around us, under conditions which do not perceptibly vary from year to year or from century to century. We have seen that every species is exposed to numerous and varied dangers throughout its entire existence, and that it is only by means of the exact adaptation of its organisation—including its instincts and habits—to its surroundings that it is enabled to live till it produces offspring which may take its place when it ceases to exist. We have seen also that, of the whole annual increase only a very small fraction survives; and though the survival in individual cases may sometimes be due rather to accident than to any real superiority, yet we cannot doubt that, in the long run, those survive which are best fitted by their perfect organisation to escape the dangers that surround them. This "survival of the fittest" is what Darwin termed "natural selection," because it leads to the same results in nature as are produced by man's selection among domestic animals and cultivated plants. Its primary effect will, clearly, be to keep each species in the most perfect health and vigour, with every part of its organisation in full harmony with the conditions of its existence. It prevents any possible deterioration in the organic world, and produces that appearance of exuberant life and enjoyment, of health and beauty, that affords us so much pleasure, and which might lead a superficial observer to suppose that peace and quietude reigned throughout nature.
The Effect under changed Conditions.
But the very same process which, so long as conditions remain substantially the same, secures the continuance of each species of animal or plant in its full perfection, will usually, under changed conditions, bring about whatever change of structure or habits may be necessitated by them. The changed conditions to which we refer are such as we know have occurred throughout all geological time and in every part of the world. Land and water have been continually shifting their positions; some regions are undergoing subsidence with diminution of area, others elevation with extension of area; dry land has been converted into marshes, while marshes have been drained or have even been elevated into plateaux. Climate too has changed again and again, either through the elevation of mountains in high latitudes leading to the accumulation of snow and ice, or by a change in the direction of winds and ocean currents produced by the subsidence or elevation of lands which connected continents and divided oceans. Again, along with all these changes have come not less important changes in the distribution of species. Vegetation has been greatly modified by changes of climate and of altitude; while every union of lands before separated has led to extensive migrations of animals into new countries, disturbing the balance that before existed among its forms of life, leading to the extermination of some species and the increase of others.