CHAPTER XXV. 



CLASS MAMMALIA.* 



Birds and Mammals are the two highest classes of Verte- 

 brates. As they have evolved along very different lines, 

 Birds possessing the air and Mammals the earth, we cannot 

 say that either class is the higher. But apart from the fact 

 that man himself is zoologically included among Mammals, 

 this class is superior to Birds in two ways, — in brain develop- 

 ment and in the relation between mother and offspring. In 

 most Mammals there is a prolonged organic connection 

 between the mother and the unborn young, and perhaps 

 Robert Chambers was right in suggesting that this prolonged 

 gestation was one of the primary factors in progress, some- 

 how connected, it may be, with the development of large 

 brains. Moreover, it is characteristic of Mammals that the 

 young are nourished after birth by their mother's milk, and 

 it has been reasonably suggested that the prolonged infancy 

 of young Mammals was one of the most important factors 

 in the evolution of gentleness. It is well to notice that one of 

 the essential conditions of the survival and success of Mam- 

 mals, and of Birds also, has been the affectionate carefulness 

 and sacrifice of the mothers. We may find in the term 

 Mammalia, which Linnaeus first applied to the class, a hint 

 of the idea that in the evolution of these forms of life, 

 the mothers led the way. 



General Survey of Mammals. — There are three grades of 

 Mammalian development. 



* In writing the systematic part of this chapter I have been especially 

 indebted to the "Introduction to the St-udy of Mammals" by W. H. 

 Flower and R. Lydekker. — (Lond., 1891.) From this valuable work 

 have been taken many of the statements of general characters and some 

 sentences within quotation marks. 



