458 THE ZOOLOGIST. 



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 coque- 

 tries 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, over- 

 power 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 sup- 

 ported 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 



■'■ Comparing the different nests to multiplied organs of a living body, as 

 e.g. the limbs of some crustaceans, which, being at first used both for walking 

 and other purposes, have now become specialized into jaws, claws, and more 

 effective legs. 



