40 HISTORY OF THE HUMAN BODY 



of the greatest importance in showing relationships are the 

 primitive Insectivora, the Mesodonta, the Condylarthra, and 

 the Creodonta, and although the existence of the first could 

 be surmised from their modern descendants, the discovery of 

 the fossil remains of the others were absolutely essential to 

 the reconstruction of the original relations between the three 

 great groups of primates, carnivores, and ungulates. It is 

 thus not surprising that the various orders of mammals have, 

 until recently, been treated like isolated groups, and that, even 

 yet, any scheme that may be offered must be looked upon as 

 provisional and liable to be modified by the bringing to light 

 of new evidence, especially that from palseontological sources. 

 It will be noticed that the branch leading to the Primates, 

 the order to which Man belongs, is represented in the dia- 

 gram as- one of the shortest and least specialized, a presentation 

 which, although opposed to the prevailing opinion, is in strict 

 accord with the facts ; since in anatomical structure these ani- 

 mals show comparatively little deviation from the primitive 

 mammalian type and do not exhibit the extreme specialization 

 displayed by the groups representing most of the other terminal 

 branches. Such aberrant orders as those of the bats, whales, 

 and horses, which have departed farthest from the original 

 mammalian environment, show in consequence the greatest 

 modifications and are thus the most specialized; certain other 

 groups, the peculiarities of which are not so striking, are still 

 greatly modified in comparison with the Primates. Thus the 

 majority of the ungulates show a reduction in the original 

 number of digits, the extremes resulting in either two, as in 

 the camels and deer, or one, as in the horse ; but the Primates, 

 together with the rodents and modern insectivores, preserve 

 the original number of five, inherited directly from the am- 

 phibians and reptiles. The teeth of ungulates are character- 

 ized by a great complexity in the folding of the enamel layer, 

 and in the number and arrangement of the cusps ; those of ro- 

 dents are specialized for the purpose of gnawing, and in the 

 Cetacea they are either secondarily reduced to the form of 

 simple cusps, all alike, or are lost altogether; the Primates, 



