142 HIGH SCHOOL ZOOLOGY. 



CHAPTER VI. 

 The Mammals. 



1. Among the classes of Vei'teb rates already studied we have 

 observed much difference as to the cai*e taken by the mother 

 of her young. In the Fishes, for example, large numbei-s of 

 eggs are produced, and for the most part left to their fate, only 

 a certain number of the fry reaching maturity. But there are 

 examples of nest-building forms among them, and of parents 

 which defend and protect their young, while there are others iu 

 which the young are retained within the body of the mother 

 until they are able to look after themselves. (I, 94.) Similar 

 diffei'ences are to be met with in the Amphibia and Reptilia as 

 to this ])oint, and we have also seen that some Birds make very 

 little provision for the safe hatching of their eggs, while in the 

 case of others, the eggs are incubated and protected until 

 the young escape, either able to fend for themselves or 

 in a condition to require further protection and feeding from 

 the mother. As an example of special adaptation to the 

 care of the young may be mentioned the Pigeons, in which the 

 glands of the crop secrete a milky fluid during the time of incu- 

 bation, which is used for the nutrition of the squabs. 



2. We meet with an analogous condition of affairs in the 

 Mammals, where certain glands of the skin are specially adapted 

 to furnish milk for the nourishment of the young. Even in 

 this group, however, we see great differences as to the condi- 

 tion in which the young are brought forth, for puppies are boi'n 

 blind and helpless, while the young of the Herbivora ax-e able 

 to i-un about shortly after bivth. If such diffex*ences are to be 



