occasion, or build one of its own. It even laid eggs in artificial nests, which Hudson placed in trees to test its propensities in this direction. (3) The common practise of stealing nests of other birds, but of holding them, as a rule, only for laying its own eggs, as illustrated to-day by the North American cowbird (Molothrus pecoris). The instincts of the intruder seem to be satisfied by "concealing" its eggs, or simply laying them against the wall of another bird's nest, and leaving them. At this stage the European cuckoo, we may suppose, not only frequently dropped its eggs on the ground, but occasionally tried to incubate them, and may have even attempted a rough nest. At this stage also the normal tendency to lay eggs at daily intervals was possibly disturbed, and the interval became irregular, with the gradual establishment of a longer rhythm.
At this point several roads would seem to be open, for the resources of nature are not limited to one course. Parasitic or non-brooding cuckoos have "chosen" one, so to speak, the brooding American species another, and if we are to accept the accounts, certain owls, which breed in the far north, successfully rear young in the short Arctic summer, with an interval of a week or more between each egg. Yet there can be little doubt that an undue lengthening of this interval would seriously interfere with nest-life in many species, and break the tendency to guard the egg. All would seem to depend upon the correlated instincts of parent and child. With an interval of from five to seven days, which has been credited to Cuculus canorus, self-brooding would be impracticable without a change in its instincts, for it migrates in July. While it is certain that the egg-laying interval was gradually extended in this bird, it is not known at what corresponding point the parasitic practise was finally established. Certain it is, however, that then as now, the egg, whether laid direct in a nest or dropped on the ground and subsequently conveyed to one, was abandoned. The American brooding cuckoos (Coccygus erythropthalmus and C. americanus), although suffering a similar disturbance in the brooding interval (of one to three clays), have adjusted these differences by another course. The young which are hatched in succession, also leave the nest in succession, when one week old, and enter upon a climbing stage which lasts a fortnight. In this way the brood is divided into two groups, and any untoward effects which might result from a marked difference in age of the nestlings, is avoided. The greatest disadvantage of such a mixture, in the nest of this species, would seem to lie in the fact that the oldest and strongest usually succeed in holding up most of the food. We may add that the American cuckoos have never advanced far beyond the first stage, as designated above, although they have suffered a disturbance in the normal rhythm of egg-production, and that the parental instincts are as strong with them as in passerine birds. The
