442 



EVOLUTION AND ANIMAL LIFE 



The animals that mate for life have the advantage over polyg- 

 amous animals, and those whose social or mating habits give 



FIG. 27o. Altricial nestlings of the blue jay, Cyanocitta c/istata. 



rise to a division of labor over those with instincts less highly 

 specialized. 



When we study instincts of animals with care and in detail, 

 we find that their regularity is much less than has been sup- 

 posed. There is as much variation in regard to instinct among 



