Page:The Zoologist, 4th series, vol 5 (1901).djvu/488

458 cover them on any part of the water, and came to the conclusion that they had abandoned it and the nest.

From the above observations, as well as those which I made last year, it may, I think, be concluded that the nest of the Great Crested Grebe is used, habitually, by the birds to pair on; so that, if it were used for no other purpose, and the eggs were laid elsewhere, it would not be a nest at all. It would, in fact, then be a "bower," or something very much resembling one—a thalamum, round and about which, in time, all the bird's coquetries might take place; whilst its subsequent gradual elaboration and ornamentation, in the case of species gifted with a higher aesthetic sense, offers no particular difficulty that I can see. Inasmuch, however, as the instinct of incubation would in all cases—we may assume—when the eggs had once been laid, overpower the primary sexual one, why should the two clash with each other, and, if they did not clash, why should not one and the same structure subserve, without inconvenience, the uses of both? First, it must be remembered that these Grebes paired on the nest, after one egg, at least, had been laid. Here, therefore, is a risk of the eggs being broken, and anything that diminished such risk would be an advantage to the species. But I have suggested another, and, as I think, a more powerful cause, by which the bower or thalamum may have become, in time, a distinct and separate structure from the true nest—as we see in the case of the Bower-birds. If I here repeat myself, somewhat, I hope I may be excused, for I wish to recall the speculations already indulged in, before proceeding to some further ones, which arise, naturally, out of them, whether supported or not by facts which I have observed, and will shortly record. Many birds, then, build more than one nest; and, if all of these nests were used as thalama for the performance of the nuptial rite, whilst only the last-made one received the eggs, then, gradually and quite naturally, two separate structures for two separate purposes might take the place of the one "contrived a double debt to pay." This would be but according to the principle of differentiation, or specialization ("specialization of parts," one might almost, by a metaphor, call it) which prevails