150 PRIMARY FACTORS OF ORGANIC EVOLUTION. 



the series, the more evident does it become that pro- 

 gressive evolution has advanced by minute increments 

 along a definite line, and that variations off this line 

 have not exerted an appreciable influence on the re- 

 sult. 



/. The Phytogeny of Man. 



In man the feet retain the pentadactyle plantigrade 

 type with scarcely grooved astragalo-tibial articula- 

 tion, which characterizes the Mammalia of the Puerco 

 epoch, and most of those of the Lower Eocene. 1 His 

 dentition is not lophodont, but is simply bunodont, like 

 that of the Phenacodontidae of the Lower Eocene. It 

 is only in the structure of the brain and the reproduc- 

 tive system that man shows an advance over the Eocene 

 type. In the former he greatly excels any mammal 

 that has appeared since ; a superiority already apparent 

 in one of his early ancestors, the anaptamorphous lemur 

 of the Lower Eocene. In the reproductive system he 

 is about on a par with the higher Artiodactyla, although 

 the male, in the persistent union of the genital and urin- 

 ary efferent ducts is not so much specialized as some of 

 the latter, where they are distinct. It is an interesting 

 fact that man displays in his dentition strong tenden- 

 cies to a greater specialization by simplification beyond 

 the ordinary quadrumanous type, by reduction in the 

 number of the true molars and incisors. Thus the M.-Ms 

 not unfrequently absent in the highest races, and some 

 families display a rudimental condition and absence of 

 the I.i. 



Much importance attaches to the composition of 

 the molar dentition. Many years ago, Owen 2 called 



IThis fact was first pointed out by myself in the Penn Monthly Magazine, 

 1875 ; see Origin of the Fittest, p. 268. 

 lOdontography, 1840-5, p. 454. 



