OF THE CLASS MAMMALIA. 



For the most part, a mammal may readily be distinguished 

 from every other animal by its external characters, even by 

 the uneducated; but not always, for to this day such 

 persons mistake a porpoise or a whale for a fish, than which 

 there cannot be a greater error in point of fact. 



Fig. 170. Porpoise, or Common Dolphin. 



389. Development of the Function of Lactation. That 

 which perhaps is the most remarkable in the history of the 

 mammals is their mode of lactation, by which is meant the 

 mode of nourishment of the young. They are viviparous, 

 and the young from birth depend immediately on the 

 nourishment they receive from the mother; unlike the 

 ovipara, which are at birth all but wholly independent of 

 their parent, in respect at least of nourishment. 



The milk intended for the nourishment of the young is 

 formed of water holding in solution, sugar of milk, casein, 

 some salts, with a little free lactic acid, holding in suspen- 

 sion globules of butter. Its qualities vary in different ani- 

 mals, and may be modified by the food of the mother. By 

 evaporation it leaves from ten to twelve solid parts of one 

 hundred. 



The secreting glands of this alimentary fluid are the 

 mammae, of which, in the male, there exist only the rudi- 

 ments. They are peculiar to the class mammals, and hence 

 their name. Their number is in relation to that of the young. 

 In apes, the elephant, goat, hare, there are two ; in the cow, 

 horses, doe, there are four ; in the cat, eight ; in the pig and 

 rabbit, ten ; in the rat, ten or twelve ; in the agouti, twelve to 

 fourteen. They vary also in their position : in the apes and 

 bats they are placed on the chest, as in woman ; in most 

 carnivora, they are situated on the abdomen as well as on the 

 thorax; and in the mare, cow, ewe, &c., they are placed 

 still further back, close to the articulations of the hinder 

 extremities. 



