320 PRESENT PROBLEMS IN EVOLUTION AND HEREDITY. 



lose our twelfth rib. The upright position, aud coasequeut transfer of 

 the weight of the abdominal viscera to the pelvis, may be considered 

 the habit associated with this reduction of the chest; at all events, in 

 the evolution of quadrux)eds there is a constant relation of increase be- 

 tween the size of the posterior ribs and the weight of the viscera, until 

 the rib-bearing vertebra' rise to twenty and the lumbars are reduced 

 to three.* It would be interesting to note the condition of the ribs in 

 some of the large-bellied tribes of Africans in reference to this point. 



The coccyx has naturally been the center of active search for the 

 missing flexible caudals. As is well known, the adult coccyx contains 

 but from three to five centers, while the embryo contains from five to 

 six. Dr. Max Bartels has made "Die geschwiinzten Menschen" the 

 subject of an exhaustive memoir upon cases of tlie reversion of the 

 tail, while Testut records all the i)rimitive tail jnuscles in various 

 stages of reversion. Watson reports that the curvatores coccygia 

 (depressores cauda^) occur only in 1 in 1,000 cases. 



This suggests a moment's digression to consider the different phases 

 of reversion. The thirteenth rib recurs by wliat Gegenbaur calls "neo- 

 genetic reversion," t for it is simply the anomalous adult development of 

 an embryonic rudiment. Under neogenetic reversions many authors 

 also include cases of the "arrested development," or persistence of an 

 embryonic condition to adult life, such as the disunited odontoid proc- 

 ess of the axis vertebra, which happens to repeat a very remote an- 

 cestral condition. I think such cases may illustrate a reversional 

 tendency, although many cases of arrested development, such as 

 anencephaly, have no atavistic significance whatever.^ More rare 

 and far more difficult to explain are the " paheogenetic reversions," in 

 wbich the anomaly, such as the supracondylar foramen, reverts to an 

 atavus so remote that the rudiment is not even represented in the 

 embryo. 



The features of skull development are primarily the increase of the 

 cranium aud 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 undoubtedly an ada]itatiou to brain 

 growth. In his valuable statistics upon the Cambridge students, 

 Galton says: "Although it is pretty well ascertained that in the 

 masses of the ijopulation tbe 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 head growth is precocious, their heads 



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



^ ■ — ■ 



' In the elephant and rhinoceros. 



\ Morph. Jahrh., Bd. vi, p. 585. 



t Anencephaly, it should be said, is iret£ueiitly associated with numerous rever- 

 sions. 



