246 A HISTORY OF BIRDS 



is precisely similar to that which obtains among the Game- 

 birds. 



This is a point of some considerable importance, since it 

 shows that, as we have reason to believe on other grounds, the 

 giant members of the Ostrich tribe have attained their present 

 conspicuous bulk comparatively recently, that is to say, smce 

 they became flightless. That they are primitive types there 

 can be no prssibility of doubt, but like other primitive types, 

 their great size is the last developmental phase in their life- 

 history, and precedes extinction. 



It is time that we turned to the opposite side of this picture 

 ■ — to the consideration of those types of nestlings which are 

 ushered into the world blind, naked and helpless. According 

 to the terms of our argument, birds were originally a strictly 

 arboreal group, and their young, like those of reptiles, were 

 extremely active from the moment they left the shell. 



Without doubt, such activity in an arboreal nursery must 

 have been attended by considerable infant mortality through 

 the young falling to the ground. Many, probably, would fall 

 through weakness ; the habit of dispersing themselves among 

 the branches of the 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. 



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 could be curtailed. The 

 Game-birds, Ducks and Geese, Rails, Cranes and Plovers, may 

 serve for ex imples of those species which have descended 

 from the trees to the ground for nesting purposes. Although, 

 as a consequence, such young have undergone considerable 

 changes in adaptation to their new environment, these changes 

 are not so striking as those which have taken place among the 

 young of the species which, retaining the ancient practice of 

 nesting in the tree tops, have adopted the alternative of cur- 

 tailing the activity of their offspring. This curtailment was 

 accomplished by reducing the amount of food-yolk enclosed 

 within the egg. As a consequence of this reduction the 

 embryonic period of development has become relatively 

 shortened, and the young accordingly emerge from the shell 

 in the helpless condition to which we have referred already. 



