throughout nature. Moreover, as the eggs would only be laid—after a full indulgence of the sexual passion—in the last nest, the incubating instinct might gradually restrain the birds—now somewhat sated—from pairing on that one; whilst the others, being used for that purpose only, would tend more and more to be built for it only, too. With regard to the multiplication of nests, we have the Wren as a familiar example of the habit, whilst my last year's observations on these same two Grebes record it in this species. Peewits are another instance, for they make a number of hollows, in all respects similar to the one in which the eggs are finally deposited, though, from their strange manner of doing this, another question arises, which I shall shortly bring forward. None of these birds are at all closely allied to the Bower-birds of Australia, but in the Thrush and the Blackbird we, at any rate, get a good deal nearer to them. With regard to the Blackbird, I have seen one clear instance of an apparently quite capricious abandonment of an almost finished nest in order to build another; nor is it in the least likely that I happened here—any more than in the case of the Grebes—to come upon a pair of very exceptional birds. It is the rarest thing, I think, speaking generally, to meet with a real exception. The appearance of it, in nine hundred and ninety-nine cases out of a thousand, marks but our ignorance. There remains the Thrush, and to this bird I paid some attention this spring, and was surprised at the number of nests which I found in different stages of construction, and which were not afterwards completed. That birds have, as a rule, any particular—or, at least, any clearly defined—object in building more than one nest, I do not myself believe; but, be that as it may, such a habit, joined to the one of pairing on the nest, appears to me to offer just that sort of foundation out of which such a state of affairs as we have with the Bower-birds might eventually arise.
But now another question arises. If a certain structure—the nest—is habitually made use of by any species of bird for pairing as well as for laying eggs in, which of these two uses are we to consider as the primary, and which the secondary one? In other words, has the bird built a thalamum which has become, in time, a nest, or a nest which has become a thalamum? This brings us to the origin of nest building, which need not,
