Darwin throughout OS is arguing against a group of opponents; and this opposing position has, I would suggest, a major influence on the particular way in which Darwin goes about his argument. (I also think that a great deal of OS's logical success is precisely that it is a very powerful argument against this position.)
So what is the position, as Darwin himself views it? Darwin actually characterizes it in several different ways. In discussing the character of domestic varieties (Chapter 1), for instance, he talks about "the immutability of the many closely allied natural species". In Chapter 2 he talks about the view that "species are immutable creations" and opposes it to "the derivative theory," i.e., the theory that specis derive from other species. Later on he talks about looking "at each species as a special act of creation." In Chapter 6 he talks abotu the belief "in separate and innumerable acts of creation". So there seem to be two points here:
(1) Species are immutable (i.e., they do not change into other species).
(2) Species are independently created.
The relation between the two appears to be that (1) is put forward as a reason for holding (2). Thus much of Darwin's argument in OS is an argument against (1); but in arguing against (1) he also sees himself as arguing against (2). This makes sense; if species are immutable in the above sense, either they have always existed (which every naturalist knows to be false) or they trace to back to independent causal originations. With his positive arguments for a theory of derivation added to the mix, this makes a solid argument. Darwin's general thesis in OS is that species do not have independent causal originations, but are, instead, derived from each other or common ancestors.
But Darwin wants to do more than this, and the reason for this is conveyed in a passage in the Introduction:
In considering the Origin of Species, it is quite conceivable that a naturalist, reflecting on the mutual affinities of organic beings, on their embryological relations, their geographical distribution, geological succession, and other such facts, might come to the conclusion that each species had not been independently created, but had descended, like varieties, from other species. Nevertheless, such a conclusion, even if well founded, would be unsatisfactory, until it could be shown how the innumerable species inhabiting this world have been modified so as to acquire that perfection of structure and co-adaptation which most justly excites our admiration. Naturalists continually refer to external conditions, such as climate, food, &c., as the only possible cause of variation. In one very limited sense, as we shall hereafter see, this may be true; but it is preposterous to attribute to mere external conditions, the structure, for instance, of the woodpecker, with its feet, tail, beak, and tongue, so admirably adapted to catch insects under the bark of trees. In the case of the misseltoe, which draws its nourishment from certain trees, which has seeds that must be transported by certain birds, and which has flowers with separate sexes absolutely requiring the agency of certain insects to bring pollen from one flower to the other, it is equally preposterous to account for the structure of this parasite, with its relations to several distinct organic beings, by the effects of external conditions, or of habit, or of the volition of the plant itself.
In other words: it is not enough to say that species are derived from species; we need to find a way for species to be derived from species that will account for the structures and adaptations we actually see. Thus, although Darwin's primary opposition is the theory of special creation, he is also trying to avoid the weaknesses of other theories of derivation. It is in this context that the theory of selections is proposed.
