80 THE ORIGIN OF MAN 



in the Triassic but at no time during the Meso- 

 zoic did these small animals take the lead among 

 organisms. Mammals, structurally the highest 

 group of animals, are warm-blooded vertebrates 

 with glands that secrete milk when the females 

 are nursing young. These glands which vary in 

 number from one to eleven pairs, are the mam- 

 mary glands or breasts, the structures from 

 which the class has taken its name, for mamma 

 means breasts. They are also present in males 

 but are rudimentary and non- functional. 



Mammals are all more or less covered with 

 hair and these growths of the skin are as charac- 

 teristic of mammals as feathers are of birds. The 

 body cavity differs from that of all other verte- 

 brates in that it is completely divided into two 

 parts by a muscular membrane, the diaphragm, 

 which separates it into a thoracic cavity contain- 

 ing the heart and the lungs, and an abdominal 

 cavity containing the remaining viscera. There 

 are two sets of teeth in most mammals, the milk 



