REFLEXES, INSTINCT, AND REASON 



441 



one in the specialization of birds and mammals. When the 

 young are cared for the percentage of loss in the struggle for 

 life is greatly reduced, the number of births necessary to the 



FIG. 274. Nest and eggs of the Rufus hummingbird, Trochihts rufus. (Photograph by 

 J. O. Snyder, Stanford University, California.) 



maintenance of the species is much less, and the opportunities 

 for specialization in other relations of life are much greater. 

 In these regards, the nest-building and home-making animals 

 have the advantage over those that have not these instincts. 



