But there are other causes that have tended to modify animals; stich as habit, use or disuse of any particular organ, food, climate, &c., and these together with the fact that a variation which appears in the parent, at any period of its existence, tends to re-appear in the offspring at the same period, will enable us to account for the metamorphoses of insects, the differences of colour in the young and the adult, the horus of sheep and cattle, &c. If to these we add that of “sexual selection,” we can see why sexes differ in organs and properties. In fact most of the facts in natural history can be explained by this theory; but there are a few which at present cannot, such as the colours of certain larvæ, which are asexual. Even these may perhaps be the effects of the mysterious and unknown laws of correlation of growth and sympathy between different parts.
We must remember that the theory of natural selection is subordinate to, and totally distinct from, that of the transmutation of species; and that if the former should be found wanting it would not effect the latter in the least degree.
The third great argument urged against the theory of transmutation of species is the geological one; and may be divided into two heads.