MAMMALS 299 



In the characteristics of intelligence and reason, man, of course, 

 stands immensely superior to all other animals, but both intelli- 

 gence and reason are too often shown by many of the other 

 mammals not to make us aware that man's mental powers 

 differ only in degree, not in kind, from those of other animals. 



Development and Life History. All animals except the 

 Monotremes give birth to free young. The Monotremes 

 produce their young from eggs hatched outside the body. 

 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 and 

 from the embryo, and a close connection between the vascular 

 systems of the parent and the embryo thus becomes established. 

 In this way the developing young derives its nourishment from 

 its mother. 



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 cow, 

 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 animals, are very 

 soon able to run about. But all are nourished for a longer or 

 shorter time by the milk secreted by the mammary glands of 

 the mother. 



Classification. 1 The mammals are usually divided into 

 eleven orders, eight of which occur in North America. The 

 small order Monotremata includes but three species of primitive 

 mammals, each representing a separate genus. They are 

 found in Australia, Tasmania and New Guinea. Their most 

 unusual and primitive characteristic is that of laying eggs. 

 After hatching from the eggs the young are nourished on milk 

 which does not issue from teats, but is poured out over the 



1 The classification adopted here is that used by Hornaday in his 

 American Natural History. While later authorities have changed many 

 of the scientific names, especially those of genera, the ones here used are 

 the most familiar and hence the most useful. 



