EVOLUTION OF THE HUMAN SPECIES 159 



but not always the animals of this order are tree-dwellers 

 in correlation with the grasping powers of the feet 

 and hands. The permanent teeth succeed a shorter 

 series of so-called ''milk teeth," and they are diverse 

 in structure, being incisors, canines, or " eye teeth," 

 premolars, and molars ; the particular numbers of each 

 kind are almost invariable throughout the order and 

 markedly different from those of other orders. The 

 number of digits is always five, and with few exceptions 

 they bear nails instead of claws. The clavicles, or 

 ''collar bones," are well developed in correlation with 

 the prehensile nature of the fore limbs ; a bony ring 

 surrounds the orbit or eye socket. Finally there are 

 two mammary glands by which the young are suckled. 

 It is because any other details of difference between 

 man and other forms are far less marked than the 

 agreements in these respects, that the human species 

 must be regarded as a primate mammalian vertebrate. 



The comparative study of the human organism as a 

 structural type has now been narrowed down to a review 

 of the various members of the order of primates. It is 

 the duty of science to arrange these organisms accord- 

 ing to the minor differences beneath the agreements in 

 major qualities, and to show how they are related in an 

 order of evolution. It will appear, when this is done, 

 that the supreme place is given to the human species 

 on account of four and only four characteristics ; these 

 are (1) an entirely erect posture, (2) greater brain devel- 

 opment, (3) the power of articulate speech, and (4) the 

 power of reason. As we are treating the human body 



