42 Studies in Animal Behavior 



rived and laid their eggs, which would have been left 

 to rot had not some birds just blundered into cud- 

 dling over them and rescued the line from extinc- 

 tion." A little reflection makes it evident that such 

 an origin is clearly impossible, and that we must go 

 back probably to the cold-blooded reptilian ances- 

 tors of the birds for the beginnings of the instinct. A 

 comparative survey of the behavior of the more 

 primitive animals toward their eggs makes it prob- 

 able that the instinct of incubation grew out of the 

 instinct to remain in or near the place where the 

 eggs are deposited for the purpose of protecting 

 them. Lying near or brooding over the eggs may 

 have afforded, even in the cold-blooded ancestors of 

 the birds, sufficient heat to make the eggs develop 

 with increased rapidity. The advantage thus accru- 

 ing to the species may have caused the protecting 

 instinct to develop further into a true instinct for 

 incubation. With the development of warm blood- 

 edness which went on during the evolution of the 

 birds from the reptiles the supplying of artificial 

 heat, at first only a means of hastening development, 

 became an indispensable condition of development. 

 Dependence upon artificial heat must have been 

 evolved pan passu with the development of the in- 

 stinct for incubation. Certainly the latter never 

 could have been developed after the former had 

 been established. 



. There is much evidence to show that the next 

 step toward the development of parental affection 



