the young depends on where the mother places her eggs, as in the case of the caterpillars of the cabbage-butterfly, we may suppose that the parent stock of the species deposited her eggs sometimes on one kind and sometimes on another of congenerous plants (as some species now do), and if the cabbage suited the caterpillars better than any other plant, the caterpillars of those butterflies, which had chosen the cabbage, would be most plentifully reared, and would produce butterflies more apt to lay their eggs on the cabbage than on the other congenerous plants. {293} This is an expansion of an obscure passage in the Essay of 1842, p. 19. However vague and unphilosophical these conjectures may appear, they serve, I think, to show that one's first impulse utterly to reject any theory whatever, implying a gradual acquirement of these instincts, which for ages have excited man's admiration, may at least be delayed. Once grant that dispositions, tastes, actions or habits can be slightly modified, either by slight congenital differences (we must suppose in the brain) or by the force of external circumstances, and that such slight modifications can be rendered inheritable,--a proposition which no one can reject,--and it will be difficult to put any limit to the complexity and wonder of the tastes and habits which may _possibly_ be thus acquired. _Difficulties in the acquirement by Selection of complex corporeal structures._ After the past discussion it will perhaps be convenient here to consider whether any particular corporeal organs, or the entire structure of any animals, are so wonderful as to justify the rejection _prima facie_ of our theory{294}. In the case of the eye, as with the more complicated instincts, no doubt one's first impulse is to utterly reject every such theory. But if the eye from its most complicated form can be shown to graduate into an exceedingly simple state,--if selection can produce the smallest change, and if such a series exists, then it is clear (for in this work we have nothing to do with the first origin of organs in their simplest forms{295}) that it may _possibly_ have been acquired by gradual selection of slight, but in each case, useful deviations{296}. Every naturalist, when he meets with any new and singular organ, always expects to find, and looks for, other and simpler modifications of it in other beings. In the case of the eye, we have a multitude of different forms, more or less simple, not graduating into each other, but separated by sudden gaps or intervals; but we must recollect how incomparably greater would the multitude of visual structures be if we had the eyes of every fossil which ever existed. We shall discuss the probable vast proportion of the extinct to the recent in the succeeding Part. Notwithstanding the large series of existing forms, it is most difficult even to conjecture by what intermediate stages very many