ALV2 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, m 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, im women, by numerous, in the lower animals, by 
