To what cause, then, may we attribute the singularities of the cuckoo? May they not be owing to the following circumstances,--the short residence this bird is allowed to make in the country where it is destined to propagate its species, and the call that nature has upon it, during that short residence, to produce a numerous progeny? The cuckoo's first appearance here is about the middle of April, commonly on the 17th. Its egg is not ready for incubation till some weeks after its arrival, seldom before the middle of May. A fortnight is taken up by the sitting bird in hatching the egg. The young bird generally continues three weeks in the nest before it flies, and the foster-parents feed it more than five weeks after this period; so that, if a cuckoo should be ready with an egg much sooner than the time pointed out, not a single nestling, even one of the earliest, would be fit to provide for itself before its parent would be instinctively directed to seek a new residence, and be thus compelled to abandon its young one; for old cuckoos take their final leave of this country the first week in July. Had nature allowed the cuckoo to have stayed here as long as some other migrating birds, which produce a single set of young ones (as the swift or nightingale, for example), and had allowed her to have reared as large a number as any bird is capable of bringing up at one time, there might not have been sufficient to have answered her purpose; but by sending the cuckoo from one nest to another, she is reduced to the same state as the bird whose nest we daily rob of an egg, in which case the stimulus for incubation is suspended. A writer in 'Nature' (vol. v., p. 383; and vol. ix., p. 123), to whom Mr. Darwin refers in the latest edition of 'The Origin of Species' as an observer that Mr. Gould has found trustworthy, precisely confirms, from observations of his own, the above description of Jenner. So far, therefore, as the observations are common I shall not quote his statements; but the following additional matter is worth rendering:--