THE STORY OF THE CHIN. 



By LOUIS ROBINSON, M.D. 



The human lower jawbone differs in a very essential 

 manner from those found among the rest of the 

 Primates — and all other vertebrates — in having its 

 lower anterior border bent downwards and forwards 



The elephant (see Figure 431) has a kind of 

 chin, and among older writers in the pre-evolutionary 

 days this fact was adduced as showing its superiority 

 to other quadrupeds. But we now know that the 



Figure 424. Atnphitherium oweni 

 (Stonesfield State). 



Figure 425. Dromatherium 

 (Upper Trias, N. Carolina). 



Figure 426. — Arsinotherium 

 (Eocene). 



so as to form a chin. Recent discoveries of the 

 remains of early men, such as the Heidelberg and 

 Piltdown jaws, have informed us that this distinctive 

 shape of the inferior maxilla has increased in a 

 marked degree since the lower stages of man's 

 existence (see Figures 500 to 504). 



I propose to discuss in the present article some of 

 the causes which appear 

 to be responsible for this 

 curious deviation from 

 type. That these causes 

 were evolutionary factors 

 of considerable potency 

 becomes fairly evident 

 when we examine further 

 into the facts. The general 

 type of the mandible among 

 terrestrial vertebrates has been curiously uniform 

 from the very earliest times, as may be seen in the 

 illustrations of mesozoic and eocene jaws (see 

 Figures 424 to 426). It is, we may say, fixed 

 or stereotyped to a remarkable degree. This makes 

 the search for evolutionary forces which have so 

 changed it in our own species all the 

 interesting. 



more 



elephant's chin is a mere degenerate remnant of the 

 long lower jaw of his ancestors, the tetrabelodon (see 

 Figure 429) and the mastodon (see Figure 430). 

 In the illustrations to which reference is made the 

 process of its downward evolution is plainly shown. 

 Another interesting example is found in the dugong 

 and its relations (see Figures 482 to 484). Here 



a little search into palaeon- 

 tology shows that this 

 apparent chin is not, like 

 the elephant's, a relic of 

 decayed functions, but that 

 it has, like that of man, 

 increased and improved 

 with the ages. As seen in 

 the illustrations the du- 

 gong's collateral ancestor, 

 the halitherium, and its big extinct relative, known as 

 Steller's sea-cow (Rhytina gigas), had "chins" also, 

 but in a less marked form. As a matter of fact the 

 downward prolongation of the mandible in these 

 animals is not a chin comparable with our own at 

 all, but is merely a kind of bony rostrum on which 

 the dugong and its relations wear their horny false 

 teeth. This structure, with its curious change of 



Figure 427. Pariasaurus. Figure 428. Inostransevia 



Figure 429. Tetrabelodon. 



There are certain apparent chins found among 

 other vertebrates, a few typical instances of which, 

 with their probable evolutionary causes, it may be 

 interesting to discuss briefly. 



Figure 430. Mastodon. 



Figure 431. Elephas 

 primigenius. 



angle, is more comparable to the bony support of 

 the flamingo's bill than to a human chin. A very 

 curious fact is that we appear to find the nearest 

 resemblance in the whole animal world, whether 



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