[A] The impotence of creationism.
[B] Natural selection acts by competition.
[C] The role of natural selection in this colorful world
[D] The delicate hierarchy of the natural system.
[E] The agency of selection can account for more cases.
[F] No leaps in natural evolution.
As each species tends by its geometrical rate of reproduction to increase excessively in number; and as the modified descendants of each species will be enabled to increase by as much as they become more diversified in habits and structure, so as to be able to seize on many and widely different places in natural selection to preserve the most divergent offspring of any one species. Hence, during a long-continued course of modification, the slight differences characteristic of varieties of the same species, tend to be augmented into the greater differences characteristic of the species of the same genus.
41. __________
New and improved varieties will inevitably displace and destroy the older, less improved, and intermediate varieties; and thus species are rendered to a large extent defined and distinct objects. Dominant species belonging to the larger groups within each class tend to give birth to new and dominant forms; so that each large group tends to become still larger, and at the same time more divergent in character. But as all groups cannot thus go on increasing in size, for the world would not hold them, the more dominant groups beat the less dominant.
42. __________
This tendency in the large groups to go on increasing in size and diverging in character, together with the inevitability of much extinction, explains the arrangement of all the forms of life in groups subordinate to groups, all within a few great classes, which has prevailed throughout all time. This grand fact of the grouping of all organic beings under what is called the Natural System, is utterly unexplainable on the theory of creation.
43. __________
As natural selection acts solely by accumulating slight, successive, favorable variations, it can produce no great or sudden modifications; it can act only by short and slow steps. We can see why throughout nature the same general end is gained by an almost infinite diversity of means, for every peculiarity when once acquired in long inherited, and structures already modified in many different ways have to be adapted for the same general purpose. We can, in short, see why nature is extravagant in variety, though not generous in innovation. But why this should be a law of nature if each species has been independently created no man can explain.
44. __________
Many other facts are, as it seems to me, explicable on this theory. How strange it is that a bird, under the form of a woodpecker, should prey on insects on the ground and that upland geese which rarely or never swim, should possess webbed feet, and so in endless other cases. But on the view of each species constantly trying to increase in number, with natural selection always ready to adapt the slowly varying descendants of each to any unoccupied or ill-occupied place in nature, these facts cease to be strange, or might even have been anticipated.
45. __________
We can to a certain extent understand how it is that there is so much beauty throughout nature; for this may be largely attributed to the agency of selection. That beauty, according to our sense of it, is not universal, must be admitted by every one who will look at some hideous bats with a distorted resemblance to the human face. Sexual selection has given the most brilliant colors, elegant patterns, and other ornaments to the males. With birds it has often rendered the voice of the male musical to the female, as well as to our ears. Flowers and fruit have been rendered conspicuous by brilliant colors in contrast with the green foliage, in order that the flowers may be readily seen, visited and fertilized by insects.
As natural selection acts by competition, it adapts and improves the inhabitants of each country only in relation to their co-inhabitants; so that we need feel no surprise at the species of any one country being beaten and supplanted by the naturalized productions from another land. The wonder indeed is, on the theory of natural selection, that more cases of the want of absolute perfection have not been detected.