contour-feathers of the body and at a much later date than in the above. So also with the nidifugous young of aquatic forms.
The accelerated development of the quills is probably a remnant of an earlier phase in the life-history before protective coloration was adopted. As we have already shown, precocious flight is attended by too many perils to prove an effective means of escape from enemies.
In conclusion we may say a word about the young of the megapodes. The eggs of the megapode are, as is well known, hatched in decaying vegetables heaps, or in hot sand, instead of being incubated by the parents. To this end the amount of food-yolk within the egg has been enormously increased, enabling the normal nestling period to be passed within the egg, the young passing through the downy stage during embryonic life, and emerging from the shell, fully fledged.
That the ancestral megapode was originally hatched in trees like the young hoatzin, there can be no doubt, since like the latter the wing of the young shows a free finger-tip and an arrested development of the outer quill feathers, characters which, as we have already seen, are direct adaptations to the peculiar locomotion of tree-climbing nestlings. We may be almost certain that the increase in the food-yolk, just referred to, did not take place until some time after the descent to the ground for breeding purposes, since the wing of the young megapode forms an exact counterpart of that of the young fowl and turkey, and their allies, whilst, had the increase taken place earlier, the wing would have resembled that of the hoatzin in the possession of large claws. The latter are present now only during embryonic life.
The increase in the food-yolk, allowing the earlier nestling stages to be passed within the shell, must be accounted for by supposing the adult megapode to have been obliged to adopt this expedient to avoid perils attendant on normal incubation, perils which may since have passed away leaving no record of their nature. A return to the normal method of incubation is now impossible, the instinct therefor having been replaced by that which induces the birds to bury their eggs and leave them to be hatched by heat other than that of their bodies.
Finally, we may compare the hoatzin with the ancient archæopteryx, and the result of such a comparison will go far to prove that the former represents the most primitive of living birds.
That archæopteryx was strictly arboreal there can be no doubt—the structure of the feet indicates this much; and its long tail is a scarcely less certain index, for such an appendage is undoubtedly but ill-adapted for ground-dwelling habits. The long hand, and the large claws thereon are, so to speak, primitive characters. The period of their greatest functional activity was during the nestling stage when the young clambered about among the branches of the trees like the young hoatzin of to-day. The species was perhaps not phylogenetically