could then plainly discern the formation of the new with the older part of the fabric. We must not conclude these remarks on nidification without alluding to Mr. Wallace's chapters on the 'Philosophy of Birds' Nests,' in his work on 'Natural Selection.' This writer is inclined to suppose that birds do not build their nests distinctive of their various species by the teachings of hereditary instinct, but by the young birds intelligently observing the construction of the nests in which they are hatched, and purposely imitating this construction when in the following season they have occasion to build nests of their own. With reference to this theory it is only needful to say that it is antecedently improbable, and not well substantiated by facts. It is antecedently improbable because, when any habit has been continued for a number of generations--especially when the habit is of a peculiar and detailed character--the probability is that it has become instinctive; we should have almost as much reason to anticipate that the nest of the little crustacean _Podocerus_, or the cell of the hive-bee, is constructed by a process of conscious imitation, as that this is the case with the nests of birds. And this theory is not well substantiated by facts because, if the theory were true, we should expect considerable differences to be usually presented by nests of the same species. Unless the construction of the nest of any given species were regulated by a common instinct, numberless idiosyncratic peculiarities would necessarily require to arise, and there would only be a very general uniformity of type presented by the nests of the same species. A more valuable contribution to the 'Philosophy of Birds' Nests' is furnished by this able naturalist when he directs attention to a certain general correlation between the form of the nest and the colour of the female. For, on reviewing the birds of the world, he certainly makes good the proposition that, as a general rule, liable however to frequent exceptions, dull-coloured females sit on open nests, while those that are conspicuously coloured sit in domed nests. But Mr. Darwin, in a careful review of all the evidence, clearly shows that this interesting fact is to be attributed, not, as Mr. Wallace supposed, to the colour of the female having been determined through natural selection by the form of the nest, but to the reverse process of the form of the nest having been determined by the colour of the female.[170]