In the introductory parts Darwin explained that “the facts which kept me longest scientifically orthodox are those of adaptation—the pollen-masses in asclepias—the mistletoe, with its pollen carried by insects, and seed by birds—the woodpecker, with its feet and tail, beak and tongue, to climb the tree and secure insects. To talk of climate or Lamarckian habit producing such adaptations to other organic beings is futile. This difficulty I believe I have surmounted.” Having then stated that the reasons which induced him to accept evolution were “general facts in the affinities, embryology, rudimentary organs, geological history, and geographical distribution of organic beings,” he proceeds to give a brief account of his “notions on the means by which Nature makes her species.” The following is an abstract of the account he gives:—
SUMMARY OF THE ESSAY.
I. The success with which selection has been applied by man in making his breeds of domestic animals and plants: and this even in ancient times when the selection was unconscious, viz. when breeding was not thought of, but the most useful animals and plants were kept and the others destroyed. “Selection acts only by the accumulation of very slight or greater variations,” and man in thus accumulating “may be said to make the wool of one sheep good for carpets, and another for cloth, &c.”
II. Slight variations of all parts of the organism occur in nature, and if a being could select with reference to the whole structure, what changes might he not effect in the almost unlimited time of which geology assures us.
III. Animals increase so fast that, but for extermination, the earth would not hold the progeny of even the slowest breeding animal. Only a few in each generation can live; hence the struggle for life, which has never yet been sufficiently appreciated. “What a trifling difference must often determine which shall survive and which perish!” Thus is supplied the “unerring power” of “Natural Selection ... which selects exclusively for the good of each organic being.”
IV. If a country were changing the altered conditions would tend to cause variation, “not but what I believe most beings vary at all times enough for selection to act on.” Extermination would expose the remainder to “the mutual action of a different set of inhabitants, which I believe to be more important to the life of each being than mere climate.” In the infinite complexity of the struggle for life “I cannot doubt that during millions of generations individuals of a species will be born with some slight variation profitable to some part of its economy; such will have a better chance of surviving and propagating this variation, which again will be slowly increased by the accumulative action of natural selection; and the variety thus formed will either coexist with, or more commonly will exterminate its parent form.” Thus complex adaptations like those of woodpecker or mistletoe may be produced.
V. Numerous difficulties can be answered satisfactorily in time. The supposed changes are only very gradual, and very slow, “only a few undergoing change at any one time.” The imperfection of the geological record accounts for deficient direct evidence of change.
VI. Divergence during evolution will be an advantage. “The same spot will support more life if occupied by very diverse forms.” Hence during the increase of species into its offspring—varieties, or sub-species, or true species, the latter “will try (only few will succeed) to seize on as many and as diverse places in the economy of nature as possible,” and so will tend to “exterminate its less well-fitted parent.” This explains classification, in which the organic beings “always seem to branch and sub-branch like a tree from a common trunk; the flourishing twigs destroying the less vigorous—the dead and lost branches rudely representing extinct genera and families.”
In a postscript he says:—
“This little abstract touches only the accumulative power of natural selection, which I look at as by far the most important element in the production of new forms. The laws governing the incipient or primordial variation (unimportant except as the groundwork for selection to act on, in which respect it is all important), I shall discuss under several heads, but I can come, as you may well believe, only to very partial and imperfect conclusions.”