both birds instinctively secure the two requisites for successful incubation—even warmth and moisture—though in different ways.
That the brush turkeys are descended from stock which possessed the instinct of incubation is rendered probable from the fact that they are gallinaceous birds, allied to the curassows, wild turkeys and grouse, all of which build some kind of a nest and brood their young at the present time. Further, the fact that the same mound is used continuously by the same birds, whether by more than one pair or not, and is added to year after year, like the aerie of an eagle, and that in the ocellated megapode, at least, the adults remain in the vicinity of their mound and tend their young after leaving it, all suggest that this mound must be regarded in the light of a nest, however modified from the typical structure. From the stage seen in the ocellated magapode, it is only a step or two to that found in others, where the parents never see their young, for which they make ample provision, any more than does a turtle or a mud-dauber wasp.
More aberrant still, but in the same direction, is the behavior of the moleo, in which as in the parasitic cuckoos, other changes have arisen, which would render self-brooding difficult if not impracticable. Their large eggs, six to eight in number, are said to be deposited at the extraordinary interval of ten to twelve days, so that a period of three months would elapse, between the laying of the first and last. Again, unlike the fowls and birds generally, no turning of the eggs during incubation is necessary.
While nothing is certainly known concerning the history of these peculiar instincts of the megapodcs, it is not unlikely that, as in cuckoos and cowbirds, they have arisen through the modification of earlier and more uniform instincts which the ancestors of all modern birds seem to have possessed in common.
