Now, granting to this hypothesis the assumption that individual cuckoos have special predilections as to the species in whose nests they are to lay their eggs, and that some of these species require to be deceived by imitative colouring of the egg to prevent their tilting it out, there is still an enormous difficulty to be met. Supposing that one cuckoo out of a hundred happens to lay eggs sufficiently like those of the North African magpies (a species alluded to by Professor Newton) to deceive the latter into supposing the egg to be one of their own. This I cannot think is too small a proportion to assume, seeing that, _ex hypothesi_, the resemblance must be tolerably close, and that the egg of the magpie does not resemble the great majority of eggs of the cuckoo. Now, in order to sustain the theory, we must suppose that the particular cuckoo which happens to have the peculiarity of laying eggs so closely resembling those of the magpie, must also happen to have the peculiarity of desiring to lay its eggs in the nest of a magpie. The conjunction of these two peculiarities would, I should think, at a moderate estimate reduce the chances of an approximately coloured egg being laid in the appropriate nest to at least one thousand to one. But supposing the happy accident to have taken place, we have next to suppose that the peculiarity of laying these exceptionably coloured eggs is not only constant for the same individual cuckoo, but is inherited by innumerable generations of her progeny; and, what is much more difficult to grant, that the fancy for laying eggs in the nest of a magpie is similarly inherited. I think, therefore, notwithstanding Professor Newton's strong opinion upon the subject, that the ingenious hypothesis must be dismissed as too seriously encumbered by the difficulties which I have mentioned. We may with philosophical safety invoke the influence of natural selection to explain all cases of protective colouring when the _modus operandi_ need only be supposed simple and direct; but in a case such as this the number and complexity of the conditions that would require to meet in order to give natural selection the possibility of entrance, seem to me much too considerable to admit of our entertaining the possibility of its action--at all events in the way that Professor Newton suggests. Therefore, if the facts are facts, I cannot see how they are to be explained. Cuckoos are not the only birds which manifest the parasitic habit of laying their eggs in other birds' nests.