474 ANIMAL LIFE-HISTORIES 



of themselves, though this chance entirely depends upon the 

 devotion of their progenitors. Such a conclusion is strengthened 

 by a study of Mammals, the highest group of Vertebrates in 

 many respects. 



LIFE-HISTORIES, &c., OF MAMMALS (MAMMALIA) 



Mammals agree with Birds in being of reptilian origin, and 

 like them are hot-blooded, as a result of the perfect adaptation 

 of their organs of circulation to air-breathing (see vol. ii, p. 426). 

 The geological record shows us that Amphibians were the first 

 backboned animals to become fitted for a life on land, where, 

 in the absence of more specialized competitors, they were long 

 dominant. But the evolution of Reptiles, still better suited to 

 terrestrial life, deprived them of their supremacy, and these in 

 their turn have been superseded by Birds and Mammals. Am- 

 phibians and Reptiles are still handicapped by the inheritance 

 from fish-like ancestors of circulatory organs originally evolved 

 in the interests of gill-breathing, and in none of them are these 

 organs fully adapted to life on land. But Birds and Mammals, 

 having finally overcome this initial difficulty, are thereby 

 profoundly influenced in many ways. Increased energy and 

 increased intelligence have resulted among other things, one 

 manifestation of the latter being found in a remarkable solici- 

 tude for the welfare of their young, which is a factor of the 

 first importance in the struggle for existence. Insects alone 

 afford a parallel, and it is especially interesting to note that in 

 them also the arrangements whereby the blood is purified are 

 highly efficient, though very different in nature from those ex- 

 isting in land Vertebrates (see vol. ii, p. 437). 



Birds, of course, are dominant in the realms of air, Bats 

 alone among Mammals entering into competition with them. 

 The fact that we ourselves belong to the Mammalia is enough 

 to justify us in describing that class as the dominant one on land, 

 though Birds flourish exceedingly, and there is still abundant 

 foothold for both Reptiles and Amphibia. The prominent posi- 

 tion which Mammals now occupy is largely bound up with two 

 features in their life-history. One is the possession of milk- 

 glands for the benefit of their young, and the other is the 

 viviparous or internal development by which the vast majority 



