THE VERTEBRATES: MAMMALS 243 



Development and life-history. All mammals except the 

 Monotremes give birth to free young. The two genera of 

 Monotremes produce their young from eggs hatched out- 

 side the body; Tachyglossus lays one egg which it carries in 

 an external pouch, while Ornithorhynchus deposits two eggs 

 in its burrow. The embryo of other mammals develops in 

 the lower portion of the egg-tube, to the walls of which it is 

 intimately connected by a membrane called the placenta. 

 (In the kangaroos and opossums, Marsupialia, there is no 

 placenta.) Through this placenta blood-vessels extend from 

 the body of the mother to the embryo, the young developing 

 mammal thus deriving its nourishment directly from the 

 parent. 



The duration of gestation (embryonic or prenatal develop- 

 ment in the mother's body) varies from three weeks with the 

 mouse, eight weeks with the cat, nine months with the stag, 

 to twenty months with the elephant. Like the birds, the 

 young of some mammals, the carnivores for example, are 

 helpless at birth, while those of others, as the hoofed mam- 

 mals, are very soon able to run about. But all are nourished 

 for a longer or shorter time by the milk secreted by the 

 mammary gland of the mother. 



Habits, instinct, and reason. Despite the wonderful ex- 

 amples of instinct and intelligence shown by many insects 

 and by the other vertebrates, especially the birds, it is among 

 mammals that we find the highest development of these 

 qualities and of reason. In the wary and patient hunting 

 for prey by the carnivora, in the gregarious and altruistic 

 habits of the herd-ing hoofed mammals, in the highly devel- 

 oped and affectionate care of the young shown by most mam- 

 mals, and in the loyal friendship and self-sacrifice of dogs 

 and horses in their relations to man, we see the culmination 

 among animals of the development of the functions of the 

 nervous system. In the characteristics of intelligence and 

 reason man of course stands immensely superior to all other 



