412! PHILOSOPHY OF ZOOLOGY. 



drupeds which live and dive much in water. The liver, 

 the renal glands, and the thyroid gland, now diminish con- 

 siderably in size. The thymous gland, which is seated un- 

 der the sternum, and ascends on each side, as far as the 

 neck, now likewise gradually diminishes in size ; and, in 

 old age, not unfrequently disappears. 



The situation of young animals at birth, in reference to 

 the locomotive powers, points out three very remarkable 

 modifications. In the first, the young at birth are com- 

 pletely formed, and capable at once of moving about, and 

 following the footsteps of the mother. These require 

 merely a regular supply of food, and protection from dan- 

 ger. In the second, the young are so imperfect, that they 

 are incapable of following the mother, and are therefore 

 carried about by her. Among these, some, as the human 

 female, carry about the young in their arms ; while, among 

 the marsupial animals, there is a ventral cavity into which 

 the young are dropped at birth, and where they are nou- 

 rished for some time. The young of these last are very 

 imperfect at birth. In the third class, the young are inca- 

 pable of following the mother, and she is equally unfit for 

 carrying them. In this case, they are deposited in a nest 

 concealed from the light, and nourished by the mother at 

 stated intervals. So far as I know, the young, in these cases, 

 are born blind ; and, in some species, the external orifice of 

 the ear is likewise closed. The maternal duties imposed 

 on animals of the first class are few, when compared with 

 those which the species of the second and third classes have 

 to fulfil in reference to cleanliness and temperature. 



The nourishment of young viviparous animals, consists 

 of milk secreted in the teats. These organs consist of nu- 

 merous glands united into a mass by cellular substance and 

 fat. The ducts gradually unite, and at last open, in the 

 nipple, in women, by numerous, in the lower animals, by 



