318 PRESENT PROBLEMS IN EVOLUTION AND HEREDITY. 



To lighten the loug catalogue of facts, gathered from many authors, 

 I shall frequently allude to habit, but will ask you to consider it for the 

 time as associational rather than causal. Pouchet says: "Man is a 

 creature of the writing table, and could only have been invented in a 

 country in which covering of the feet is universal;" he should have 

 added the "eating table." From the average man our fashions and 

 occupations demand the play of the forearm and hand, the independent 

 and complex movements of the thumb and finger; the outward turning 

 of the foot in walking. These are some of the most conspicuous features 

 of modern habit. 



The skeletal variations* — In a most valuable essay by Arthur Thom- 

 son upon "The Influence of Postiue on the Form of the Articular Sur- 

 faces of the Tibia and Astragalus in the Different Eaces of Man and 

 the Higher Apes,"t we find clearly brought out the distinction between 

 congenital variations and tliose which may be acquired by prolonged 

 habits of life. It is perfectly clear from this investigation that certain 

 racial characters, such as " platycnemism " or flattened tibia, which 

 have been considered of great importance in anthropology, may prove 

 to be merely individual modifications due to certain local and temporary 

 customs. Thomson's conclusions are that the tibia is the most variable 

 in length and form of any long bone in the body. Platycnemia is most 

 frequent in tribes living by hunting and climbing in hilly countries, 

 and is associated with the strong development of the tibialis posticus. 

 The great c<mvexity of the external condyloid surface of the tibia in 

 savage races appears to be developed during life by the frequent or 

 habitual knee flexure in squatting; it is less developed where the tibia 

 has a backward curve, and is independent of platycnemia. Another 

 product of the s(iuatting habit is a facet formed upon the neck of the 

 astragalus by the tibia. This is very rare in Europeans; it is found in 

 the gorilla and oraug, but rarely in the chimpanzee. We must there- 

 fore be on our guard to distinguish between congenital or hereditary 

 skeletal characters which are fundamental, and "acquired" skeletal 

 variations which may not be hereditary. The latter are of question- 

 able value in tracing lines of descent, if not actually misleading; on 

 the other hand, the teeth, as shown by Cope in his essay on "Lemurine 

 reversion in human dentition," have distinct racial patterns and are 

 reliable indices of consanguinity, because their form can not be modified 



during life. 



The main features of present evolution in the backbone are the elab- 

 oration of the spines of the cervical vertebra?, the increase of the spinal 

 curvatures, the shortening of the centra of the lumbar vertebrae and 



*For recent general articles, see Blaucliard, ^'L'Atavisme chez rHomrae," Bev. de 

 Anthro})., 1885, p- -l^B; and Baker, ''The Ascent of Man," Proceeding.^ of the American 

 Associatioi) for the Advancement of Science, 1800. Also, Smithsonian Reim-t for 1890, p. 



447. 



\ Journal of Anatomy and PhysioJocju, 1889, p. 017. 



