1892.] The Contemporary Evolution of Man. 465 



memoir upon cases of the reversion of the tail, while Testut 

 records all the primitive tail muscles in various stages of 

 reversion. Watson reports that the curvatures eoccve;ia 

 (-depressores caudse) only occur in 1 in 1000 cast s. 



This suggests a moment's digression to consider the ditl'er- 

 ent phases of reversion. The 13th rib recurs by u 

 baur 1 calls " neogenetic reversion," for it is simply the anom- 

 alous adult development of an embryonic rudiment Under 

 neogenetic reversions many anthem also include eases of the 

 " arrested development," or persistence of an embryonic con- 

 dition to adult life, such as the disunited odontoid process of 

 the axis vertebra, which happens to repeat a very remote 

 ancestral condition. I think such cases may illustrate a rever- 

 sional tendency, although many case.- of arrested development. 

 such as anencephaly, have no atavistic significance whatever. 2 

 More rare and far more difficult to explain are the " paleogen- 

 etic reversions," in which the anomaly, such as the supracon- 

 dylar foramen, reverts to an atavus so remote that the rudi- 

 ment is not even represented in the embryo. 



The features of skull development are primarily the increase 

 of the cranium and the late closure of the cranial sutures in 

 contrast with the more complete and earlier closure of the 

 facial sutures. 



So far as I can gather this seems to be another region where 

 the white and colored races present reversed conditions ; the 

 early closure and arrest of brain development in the negroes 

 is well known; the later closure among the whites is undoubt- 

 edly an adaptation to brain growth. In his valuable statistics 

 upon the Cambridge students Galton says: "Although it is 

 pretty well ascertained that in the masses of population the 

 brain ceases to grow after the age of nineteen, or even earlier, 

 it is by no means the case with "university students. In high 

 honor men headgrowth is precocious, their heads predominate 

 over the average more at nineteen than at twenty-five." 



Many of the cases of arrested closure of facial sutures are 

 reversional, as they correspond with the adult condition of 



^orph. Jahrb., Bd. vi, p. 585. 



