Still, when so great an authority as Professor Newton expresses himself satisfied that there is a marked _tendency_ to such imitation, which in some cases leads to extraordinary variations in the colouring of the cuckoo's egg, the alleged fact becomes one which demands notice. The question, of course, immediately arises, How is it conceivable that the fact, if it is a fact, can be explained? We cannot imagine the cuckoo to be able consciously to colour her egg during its formation in order to imitate the eggs among which she is about to lay it; nor can we suppose that having laid an egg and observed its colouring, she then carries it to the nest of the bird whose eggs it most resembles. Professor Newton suggests another theory, which he seems to think sufficient, but which I confess seems to me little more satisfactory than the impossible theories just stated. He says:-- Only one explanation of the process can, to my mind, be offered. Every person who has studied the habits of birds with sufficient attention will be conversant with the tendency which certain of those habits have to become hereditary. It is, I am sure, no violent hypothesis to suppose that there is a very reasonable probability of each cuckoo most commonly placing her eggs in the nest of the same bird, and of this habit being transmitted to her posterity. Now it will be seen that it requires but only an application to this case of the principle of 'natural selection,' or 'survival of the fittest,' to show that if my argument be sound, nothing can be more likely than that, in the course of time, that principle should operate so as to produce the facts asserted, the eggs which best imitated those of particular foster-parents having the best chance of duping the latter, and so of being hatched out.