no POPULAR SCIENCE MONTHLY. 



present purpose. They show at least that 'sacrifice of the parents on 

 behalf of the young' has not been uniformly rewarded 'in the im- 

 provement ... 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 pos- 

 sible 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 cer- 

 tain 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 con- 

 cealment 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 ob- 

 viously direct responses to the increased need of escaping enemies, and 

 hence we find these devices have been universally adopted. It is 



