THE GENEALOGY OF ANIMALS 91 



and are often called sea-cows. They resemble 

 whales in many respects, and are sometimes 

 classed with them. They are plant-eaters ex- 

 clusively, and are found grazing along the bottoms 

 of tropical estuaries and rivers. They have tiny 

 eyes, teeth fitted for grinding (not spike-like as in 

 the whales), and a strong affection for their young, 

 the mother, when pursued, often carrying her little 

 one under her flippers. An immense sirenian, 

 known as Steller's manatee, was discovered on 

 the Behring Islands, along the Kamschatka coast, 

 in 1741. Twenty-seven years afterwards not one 

 of them was left, all having been murdered by the 

 Russian sailors. The sirenians are probably de- 

 generate forms of land quadrupeds, having lost 

 their hind-limbs and developed the fish-like shape 

 in adapting themselves to aquatic conditions. 

 They appear first in the Eocene Age. 



Among the most interesting derivatives of the 

 herbivorous marsupials, because the most aberrant, 

 are the whales. They are true mammals have 

 warm blood, breathe the air with lungs, and suckle 

 their young like other mammals. But, like the 

 sirenians, they live in the surface of the waters, 

 and have flippers and a fish-like tail and form. 

 They differ from the sirenians, however, in being 

 carnivorous, in having inguinal instead of pectoral 

 milk glands, and in being structurally less like 

 quadrupeds. They probably degenerated from 

 land quadrupeds during the Jurassic period, and, 

 owing to their longer residence in the waters, have 

 become further removed from the quadrupedal 



