EVOLUTION OF THE HUMAN SPECIES 157 



fishes, amphibia, reptiles, birds, and mammals, each class 

 distinguished by certain definite characters in addition 

 to the vertebral column. The fishes have gills and 

 scales ; amphibia of to-day are scaleless, and they are 

 provided with gills when they are young and lungs as 

 adults ; reptiles have scales and lungs ; birds are warm- 

 blooded and feathered ; while mammals are warm- 

 blooded and haired. Is the human species a unique 

 kind of vertebrate, or does it find a place in one of these 

 classes ? The occurrence of hair, of a four-chambered 

 heart which propels warm blood, of mammary glands, 

 and of other systematic characters marks this species 

 as a kind of mammal and not as a vertebrate in a section 

 by itself. 



The members of the class mammalia differ much 

 among themselves ; and now that we recognize clearly 

 that man is a mammalian vertebrate, the next question 

 is whether an order exists to which our type must be 

 assigned, or whether we have at last reached a point 

 where it is justifiable to establish an isolated division 

 to contain the human species alone. We are familiar 

 with many representatives of different mammalian 

 orders and with the kind of structural characteristics 

 that serve as convenient distinctions in denoting their 

 relationships. Horses and cattle, sheep, and goats and 

 pigs resemble one another in many respects besides their 

 hoofs, and they form one natural order ; the well- 

 developed gnawing teeth of rats and rabbits and 

 squirrels place these forms together in the order roden- 

 tia ; the structures adapting their possessors for a flesh- 

 eating and predatory life unite the tribes of the lion, 

 wolf, bear, and seal, in the order carnivora. Among 



