MAN A MAMMAL 13 



as they have in the past, we may expect to have, 

 in the course of generations, men with foreheads 

 reaching regularly to the occiput. Most animals 

 lay eggs. Man does not. Like the dog, the 

 horse, the squirrel, and the bat, man is viviparous, 

 the eggs hatching within the parental body. 

 Human young are born helpless, and are sus- 

 tained during the period of their infancy by the 

 secretions of the milk glands. So are all the sons 

 and daughters of mammals. Whether they come 

 into the world among the waters or among the 

 desert sands, in the hollow of a tree, in a hole in 

 the earth, or in a palace, the children of mammals 

 are frail and pitiful, and they survive to grow and* 

 multiply only because they are the object of the 

 loving and incessant sacrifices of a mother. 



Mammals are distinguished from all other 

 animals by the possession of two kinds of skin 

 glands — the sweat glands and the oil glands — and 

 by the development of certain of these glands in 

 the female into organs for the nourishing of the 

 young. Among reptiles and birds the lower jaw 

 is suspended from the skull by a bone called the 

 quadrate bone. Among men and other mammals 

 the lower jaw is joined directly to the skull, the 

 quadrate bone becoming, in the vicissitudes of 

 evolution, the hammer (malleus) of the mammalian 

 ear. Man has a four-chambered heart — two reser- 

 voirs which receive, and two pumps which propel, 

 the scarlet waters of the body. Fishes have two- 

 chambered hearts ; frogs and most reptiles have 

 three-chambered hearts; all mammals and birds 



