This would be an excellent argument in support of the theory, were it not for one expression, upon which the whole value of the argument seems to me to depend. What is meant by the expression “once successfully deposited”? Does the Cuckoo ever revisit a nest in which she has placed an egg, and satisfy herself that her offspring is hatched and cared for? If not (and I believe such an event is not usual, if indeed it has ever been known to occur), then nothing has been gained by the selection of a Reed Wren’s or Titlark’s nest (as the case may be), and the Cuckoo can have no reason for continuing the practice of using the same kind of nest from one season to another.
While admitting therefore the tendency which certain habits have to become hereditary in certain animals, I feel compelled to reject the application of this principle in the case of the Cuckoo, on the ground that it can only hold good where the habit results in an advantage to the species, and in the present instance we have no proof either that there is an advantage, or, if there is, that the Cuckoo is sensible of it.
Touching the question of similarity between eggs laid by the same bird, Professor Newton says:—“I am in a position to maintain positively that there is a family likeness between the eggs laid by the same bird” (not a Cuckoo) “even at an interval of many years,” and he instances cases of certain Golden Eagles which came under his own observation. But do we not as frequently meet with instances in which eggs laid by the same bird are totally different in appearance? Take the case of a bird which lays four or five eggs in its own nest before it commences to sit upon them—for example, the Sparrow-Hawk, Blackbird, Missel-Thrush, Carrion Crow, Stone Curlew, or Black-headed Gull. Who has not found nests of any or all of these in which one egg, and sometimes more, differed entirely from the rest? And yet in each instance these were laid, as we may presume, not only by the same hen, but by the same hen _under the same conditions_, which can be seldom, if ever, the case with a Cuckoo.
Looking to the many instances in which eggs laid by the same bird, in the same nest, and under the same circumstances, vary _inter se_, it is not reasonable to suppose that eggs of the same Cuckoo deposited in different nests, under different circumstances, and, presumably, different conditions of the ovary, would resemble each other. On the contrary, there is reason to expect they would be dissimilar. Further, I can confirm the statement of Mr. Dawson Rowley, who says:[103] “I have found two types of Cuckoo’s eggs, laid, as I am nearly sure, by the same bird.”