present purpose. They show at least that 'sacrifice of the parents on behalf of the young' has not been uniformly rewarded 'in the improvement ... in all the faculties of the race..' More than this, they seem to me to show that this factor has had little or nothing to do with either inclination or structural development.
The real explanation of the matter seems rather to turn upon a question of expediency, designed, so to speak, to reduce infant mortality.
We shall show presently, on evidence well nigh incontrovertible, that the nidifugous condition is indeed a primitive one, but associated with a strictly arboreal habitat. This is an important point, as the nidifugous condition is commonly regarded as peculiar to, and possible only in, a terrestrial habitat. Let us assume for the moment that the former is an established fact.
One great disadvantage attendant on precocious development of the young whose nursery is the tree top is obvious the nestlings would be constantly in danger of falling to the ground, and a large number would indeed meet this fate. Some would fall through weakness, the habit of dispersing themselves among the branches of trees in which the nest was placed resulting in a loss of regular food supply, owing to the difficulty of being on the spot when the parents returned with food. Thus the more sedentary members of the family would stand the best chance of being regularly fed, but among these the danger of falling by accident would be an ever present one. Once on the ground it is probable they would perish speedily, for it is almost certain that the earliest birds were entirely arboreal, and either would not or could not seek for lost offspring amid the thick undergrowth.
Now two courses were open whereby this infant mortality could be reduced. Either the eggs could be deposited on the ground, or the activity of the young curtailed. The game-birds, ducks and geese, rails, cranes and plovers may serve for examples of those species which have descended from the trees to the ground for nesting purposes, and although, as a consequence, the young have undergone considerable modifications in adaptation to the new environment, these changes are not so striking as those which have taken place among the young of the tree-dwelling species to be described presently.
The modifications which we should expect to find in the offspring of those species which, instead of curtailing the activity of the young, descended to the ground to breed, would be (1) peculiar habits of concealment aided by protective coloration; and (2) a reduction in the size of the wings and feet, now no longer required solely as grasping organs.
Protective coloration and peculiar habits of concealment are obviously direct responses to the increased need of escaping enemies, and hence we find these devices have been universally adopted. It is
