The evidence brought forward by Mr. Darwin in support of his hypothesis is of three kinds. First, he endeavours to prove that species may be originated by selection; secondly, he attempts to show that natural causes are competent to exert selection; and thirdly, he tries to prove that the most remarkable and apparently anomalous phaenomena exhibited by the distribution, development, and mutual relations of species, can be shown to be deducible from the general doctrine of their origin, which he propounds, combined with the known facts of geological change; and that, even if all these phaenomena are not at present explicable by it, none are necessarily inconsistent with it.
[T. H. Huxley, "The Origin of Species," Lay Sermons, Addresses, and Reviews. Macmillan (London: 1880) p. 293.]
This is a fairly good summary of Darwin's argument. In this post I will be looking at Huxley's First.
Darwin opens his argument in OS with a discussion of Variation under Domestication; according to Darwin in the Introduction, the reason for this is that we may "see that a large amount of hereditary modification is at least possible; and, what is equally or more important, we shall see how great is the power of man in accumulating by his Selection successive slight variations." The ultimate point of this argument will be the recognition that on this point, as man does, so also nature does. As he will neatly put the matter in the introduction to a later work, The Variation of Animals and Plants under Domestication:
It can hardly be maintained that during the many changes which this earth has undergone, and during the natural migrations of plants from one land or island to another, tenanted by different species, that such plants will not often have been subjected to changes in their conditions analogous to those which almost inevitably cause cultivated plants to vary. No doubt man selects varying individuals, sows their seeds, and again selects their varying offspring. But the initial variation on which man works, and without which he can do nothing, is caused by slight changes in the conditions of life, which must often have occurred under nature. Man, therefore, may be said to have been trying an experiment on a gigantic scale; and it is an experiment which nature during the long lapse of time has incessantly tried. Hence it follows that the principles of domestication are important for us.
We are not yet quite to this full point here; what we are looking at here is simply the lead up that will eventually get us to this conclusion. Darwin is not, as he is sometimes portrayed, simply making an analogy. He is identifying a vera causa, and analyzing it so as to determine its essential characteristics. In doing so he establishes a number of important points: (1) At the time of his writing, the laws and causes governing variation and inheritance are either not known or only "dimly seen"; (2) In cases of domestication it can often be difficult to distinguish varietis and species; (3) In domestication, adaptations are created when "nature gives successive variations" and "man adds them up in certain directions useful to him" (he calls this "accumulative selection"); (4) "The great power of this principle of selection is not hypothetical," i.e., it is a vera causa; (5) This accumulative selection can occur unconsciously; (6) This unconscious selection produces "slow, varying, and insensible changes" in a way that is more efficient than methodical selection. (5) and (6) are very important, because by establishing these, Darwin is able to bridge over into natural selection. Unconscious selection, far more than the methodical selection we usually treat as synonymous with artificial selection, approximates what happens in nature.
But to establish this point, Darwin first has to show the characteristics of Variation under Nature; and this is the subject of Chapter II of OS. This is somewhat tricky; as Darwin notes, this can really only be done properly by giving "a long catalogue of dry facts". Since that's out of the question for a work like OS, he can only touch on the basics here, promising a more thorough treatment in a future work. (This is one of several points at which Darwin shows just how conscientious he can be about the construction of his argument.) The primary point Darwin wishes to draw from his basic treatment, besides the (already rather obvious) fact that species in nature vary, is that in nature it can be immensely difficult to make any distinction between varieties and species, and that by regarding varieties as incipient species, we can make some sense of this. If this latter point is to fly, however, we need an adequate account of Divergence of Character. Darwin turns to this in the next part of his argument, to which I will soon turn.
