242 



THE AMERICAN NATURALIST 



[Vol. L 



is to be expected, it seems to me, that during a general 

 contraction in its size, the superior margin would retract 

 more rapidly than the inferior. I am inclined to think 

 that the chin is the persistent lower margin of our large 

 ancestral jaw. This margin has become retracted more 

 slowly than the upper margin, and therefore juts out into 

 space (Fig. 6). 



A difficulty immediately suggests itself. If a human 

 chin results from reduction in the size of the jaw, wher- 

 ever in different species of animals jaws have become re- 

 duced, we ought logically to find chins. One striking case 

 can be cited in line with this suggestion. The case of 

 the elephant suggests itself at once, and very clearly. 

 We know that his jaw bones are the result of a remark- 

 able retraction. The process is one of the most pic- 

 turesque that we know about. 4 We duly find in the geo- 

 logically recent elephants, and especially in the living 

 species, a tremendous chin (Fig. 7). Whether still other 



cases of chins resulting from retraction could, or could 

 not, be cited, I do not know. I very strongly suspect, how- 

 ever, that a thorough knowledge of paleontology would 

 put one in position to cite a considerable number, though 



possibly few cases would be so clear as that of the ele- 

 phant. I can not resist the feeling that in some such 

 process we have the explanation, not only of human chins, 

 but the chins of other animals as well. 



3 A fact mentioned by MacCurdy in the Smithsonian Report for 1909 



(page 570). 



* Interestingly described by Sir Ray Lankester, "Extinct Animals." 



