126 THE INFANCY OF ANIMALS 



the nidicolous type, which, ushered into the world blind, 

 naked, and helpless, stand in such strong contrast with 

 those just described. According to the terms of our 

 argument, birds were originally strictly arboreal creatures, 

 and their young, like those of reptiles, were active from 

 the moment they left the shell. 



The evidence in support of this condition seems to 

 admit of little doubt, but it is plain that such activity 

 must have been attended with very considerable infant 

 mortality through the young falling to the ground. A 

 not inconsiderable number would perish through weak- 

 ness ; the habit of dispersing among the branches of the 

 nesting tree resulting in irregular and insufficient feeding 

 of those which had strayed too far to meet the parents 

 returning with food. Thus would begin the initiation 

 of the new type of nestling, the struggle for existence 

 weeding out the more active individuals and favouring 

 the more sedentary. 



In other words we can trace the action of natural 

 selection along two divergent paths. In the one case 

 a certain number of species, from one cause or another, 

 seem to have been driven to shift their nurseries from 

 the forests to open ground. Herein the activity of the 

 young was further increased, resulting in types such as 

 the nestlings of the gallinaceous birds, rails, cranes, and 

 plovers, all of which, in the course of time, have undergone 

 considerable transformation in different directions in 

 response to the needs of their several environments. In 

 the other, wherein the arboreal nurseries were retained, 

 a process of selection has taken place whereby a curtail- 

 ment of the activity of the young has been brought about. 

 And this has beein accomplished by reducing the amount 

 of food yolk enclosed in the egg. 



