Now let us suppose that the ancient progenitor of our European cuckoo had the habits of the American cuckoo, and that she occasionally laid her egg in another bird's nest. If the old bird profited by this occasional habit through being able to migrate earlier, or through any other cause; or if the young were made more vigorous by advantage being taken of the mistaken instinct of another species than when reared by their own mother, encumbered as she could hardly fail to be by having eggs and young at the same time;[175] then the old birds or the fostered young would gain an advantage.[176] The instinct would seem to be a very old one, for there are two great changes of structure in the European cuckoo which are manifestly correlated with the instinct. Thus, the shape of the young bird's back has already been noted; and not less remarkable than this is the small size of the egg from which the young bird is hatched. For the egg of the cuckoo is not any larger than that of the skylark, although an adult cuckoo is four times the size of an adult skylark. And 'that the small size of the egg is a real case of adaptation (in order to deceive the small birds in whose nests it is laid), we may infer from the fact of the non-parasitic American cuckoo laying full-sized eggs.' Yet, although the instinct in question is doubtless of high antiquity, there have been occasional instances observed in cuckoos of reversion to the ancestral instinct of nidification; for, according to Adolf Müller, 'the cuckoo occasionally lays her eggs on the bare ground, sits on them, and feeds her young.' In 'Nature' for November 18, 1869, Professor A. Newton, F.R.S., has published an article on a somewhat obscure point connected with the instincts of the cuckoo. He says that Dr. Baldamus has satisfied him, by an exhibition of sixteen specimens of cuckoos' eggs found in the nests of different species of birds, 'that the egg of the cuckoo is approximately coloured and marked like those of the bird in whose nest it is found,' for the purpose, no doubt, of deceiving the foster-parents. Professor Newton adds, however:-- Having said this much, and believing as I do the Doctor to be partly justified in the carefully worded enunciation of what he calls a 'law of nature,' I must now declare that it is only 'approximately,' and by no means _universally_ true that the cuckoo's egg is coloured like those of the victims of her imposition, &c.