496 The Outline of Science 



and the fish-eating seals, and others with a catholicity of 

 appetite like badger and otter. A harvest-mouse only weighs 

 about a halfpenny, an elephant's tusk may weigh 188 pounds. 

 The Pigmy Shrew has a body under 2 inches in length, a whale 

 may attain to 60 feet. A common shrew seems often to die in 

 the year of its birth ; an elephant may be more than a centenarian. 

 But we need not go further ; it is plain that there is extraordinary 

 variety among mammals. This raises the question, what have 

 they all in common? 



General Characters of Mammals 



Mammals are quadrupeds, except that the whales and sea- 

 cows have lost all but vestiges of the hind-limbs, and perhaps 

 another saving clause should be inserted for kangaroos, jerboas, 

 and higher apes, which are more or less bipeds. In most mammals 

 there is a distinct neck and a distinct tail, but the neck is prac- 

 tically obliterated in whales, and the tail is often much reduced (as 

 in bear and rabbit) or practically absent (as in the higher apes). 



Hairs are never entirely absent, for even in whales they are 

 present in early stages of life and some, very richly innervated, 

 often persist on the lips. The mammalian skin shows sweat- 

 glands which get rid of surplus water and some waste-products, 

 sebaceous glands which keep the fur sleek ( absent in whales ) , and 

 milk -glands which are normally functional in the females only. 



In mammals only is there developed a midriff or diaphragm 

 a muscular sheet separating the chest cavity (containing heart 

 and lungs) from the abdominal cavity (containing the stomach 

 and other viscera). This midriff falls and rises in the breathing 

 movements, and is of great importance in increasing and then de- 

 creasing the chest cavity, and thus helping the entrance and exit 

 of air from the lungs. 



Mammals have many skeletal peculiarities which separate 

 them off from all other backboned animals. The vertebra? ( back- 

 boned-bodies) and the long bones have terminal caps which ossify 



