128 THE INFANCY OF ANIMALS 



resort to trees for a nursery. But in these cases the 

 young are evidently on the way to become nidicolous. 

 In appearance they are not to be . distinguished from 

 nidifugous types, but in behaviour they are nidicolous. 



These instances seem the more remarkable because they 

 stand in such striking contrast with what obtains among 

 their immediate allies. But as a matter of fact there are 

 a number of instances wherein nidifugous are in process 

 of transformation into nidicolous types — such as, for 

 example, the gannets, cormorants, pelicans and their allies, 

 which in the early stages of development are now ab- 

 solutely helpless, and some are losing their downy covering 

 as in typical nidicolous birds. While the pelicans always 

 nest upon the ground, some species both of gannets and 

 cormorants nest in trees, either sporadically or constantly, 

 as the conditions of the environment may determine. 

 Various intermediate stages in this process of " hobbling " 

 may be studied in the case of those species which, having 

 precocious young, nest in colonies, often of vast size, or 

 on the ledges of precipitous cliffs. 



The reduction of the food-yolk, and the consequent 

 earlier hatching of the young of such species, are decidedly 

 advantageous changes. The gain of such curtailment 

 of movement is, to species which nest in colonies on 

 the ground, obvious. Active young, hatched in colonies 

 affording free space wherein to roam, would soon become 

 lost amid the general crowd of nestlings, and hence speedily 

 starve — even if the adults had acquired the habit of in- 

 discriminately feeding any nestling of the colony which 

 clamoured for food. In the case of cliff-breeding species 

 like guillemots and razor-bills, and many species of gulls, 

 this quiescence saves an enormous mortality through falls 

 from the diffs. 



