492 ELEMENTARY BIOLOGY 



vertebrates known as mammals, because, like the mammals in 

 general, the human species suckles its young and has a hairy 

 outgrowth on the skin. And, of the ten or a dozen orders into 

 which the class Mammalia is usually divided (see p. 486), the 

 highest, or primates, seems best to agree with what we know 

 of our own structure. 



In the matter of the limbs, all primates have the thigh quite 

 free from the body, and the hand and foot have five distinct 

 digits, with the first one (inner) usually opposable, thus con- 

 stituting a grasping organ. The soles of the feet (or hands) 



FIG. 255. Skulls of primates 



Compare the size of the face and jaw with the size of the brain box. Note the appear- 

 ance in man of a distinct chin and a nose bridge. /, lemur ; 2, gibbon ; j, man 



come down flat on the ground. The lemurs are obviously less 

 like man than are the monkeys, and these less than the apes. 

 511. Man and other primates. The important differences 

 between man and his nearest living relatives are found in the 

 erect walk, the differentiated appendages, a distinct chin, de- 

 cidedly larger brain, and articulate speech. Animals other than 

 man are able to get up on their hind legs for longer or shorter 

 periods, but none of them ever acquire the definite, erect walk 

 that all human beings develop (Figs. 253, 254). 



It has been pointed out that acquiring the habit of walking alto- 

 gether on their hind legs gave the ancestors of the human race an 

 opportunity to free their arms and hands for other activities, and that 

 therefore it became possible to develop these organs to higher skill. 

 We must be on our guard against assuming that the evolution of man 

 (or of any other species) proceeded by the hereditary transmission 

 from generation to generation of the effects of practice or experience. 



