338 EVOLUTION AND ANIMAL LIFE 



powerfully armed than the female. In the polygamous group 

 to which the hen, turkey, and peacock belong, the males pos- 

 sess the display of plumage, and the structures adapted for 

 fighting, with the wdll to use them. 



The protection of the young is the source of many adaptive 

 structures as well as of the instincts by which such structures 

 are utilized. In general those animals are highest in develop- 

 ment, with the best means of holding their own in the struggle 

 for life, that take best care of their young. Those instincts 

 which lead to home building are all adaptations for preserving 



Fig. 198. — The snake, Ichthyophis glutinosus, with egg case carried in coils of the body, 



(After Goebel and Selenka.) 



the young. Among the lower or more coarsely organized birds, 

 such as the chicken, the duck, and the auk, as with the reptiles, 

 the young animal is hatched with Avell-developed muscular 

 system and sense organs, and is capable of running about, and, 

 to some extent, of feeding itself. Birds of this type are known 

 as prsecocial, while the name altricial is applied to the more 

 highly organized forms, such as the thrushes, doves, and song 

 birds generally. With these the young are hatched in a wholly 

 helpless condition, with ineffective muscles, deficient senses, 

 and dependent wholly upon the parent. The altricial condition 

 demands the building of a nest, the establishment of a home, 

 and the continued care of one or both of the parents. 



The very lowest mammals known, the duck bills (Mono- 

 tremes) of Australia, lay large eggs in a strong shell like those 

 of a turtle, and these they guard with great jealousy. But 



