PARENTAL CARE AND NEST-MAKING 317 



As the eggs of birds are not carried about after they are 

 liberated, and as there is no viviparity in birds, an expedient 

 is to bury them in the ground — as ostriches and some 

 mound-birds do. Some reptiles do likewise. The mother 

 crocodile sometimes lingers near the place where she has 

 buried the eggs and digs them up when the young are ready 

 to hatch out. This lingering near the spot points the way to 

 brooding, and that again begins with reptiles, as is illustrated 

 by the python. 



But brooding on the ground is tedious and risky, and 

 there is a long inclined plane of nests which give the parent 

 bird more comfort and safety and make the hatching out more 

 successful. A prolongation of development and of the 

 period of infancy was possible in a nest safely hidden in a 

 tree, and it may have been that this favoured the evolution 

 of a large and plastic brain — not too heavily endowed with 

 instincts to be edticahle. 



But greater success in rearing the family make a smaller 

 clutch possible, and a tightening of the cords of love. 

 Emotional bonds were forged — a premium was put on the 

 sympathetic type. Family life began, often with some 

 education, occasionally with some short-lived comradeship. 

 And some birds went beyond this to the level of sociality. 



