THE VERTEBRATES AND THEIR ENVIRONMENT 



29 



cases, as with the mare's colt, the young can stand and run almost 

 at birth. In others, as in dogs and cats, there is a period of greater 

 dependence. Adult mammals are adapted for a wide range of 

 terrestrial conditions. The chamois, mountain sheep, and goats 

 inhabit the heights; deer and wolves, the forests and open country; 

 members of the horse and cattle families, the great plains; other 

 mammals range along the shores of streams, lakes, or oceans. 

 Among the most interesting cases are those of such mammals as 

 whales, porpoises, and seals, which show varying degrees of adap- 

 tation to life in the water. The same peculiarity is shown, in a 



Fig. 15. — The duckbill or platypus, Ornithorhyncus anatinus, an Australian 



mammal that lays eggs. 



(From Parker and Haswell, "Textbook of Zoology," copyright, 1921, by Macmillan 

 & Co., Ltd., reprinted by permission.) 



lesser degree, by beavers, muskrats, and many others. These 

 animals are obviously mammals; they have hair and give birth to 

 well-developed young which are nourished by means of mammary 

 glands. They are able, in extreme cases like that of the whales, 

 to spend their entire Kves in the water, coming up at intervals for 

 air when not swimming at the surface. Others, hke the fur seal, 

 may spend most of their lives in the water save for the breeding 

 seasons. The fur seals arrive at the seal islands of Bering Sea early 

 in the summer; they give birth to their young on land at the so- 

 called "rookeries" (Fig. 16); the young pass their early life in 

 and out of the water; and in the late summer the seals " haul back 

 to the sea and no man knows their track," although they return 

 to the beaches year after year. A seal is a very clumsy animal on 



