THE STORY OF THE CHIN.! 
By Lovis Roxsryson, M. D.? 
[With 12 plates.] 
The human lower jawbone differs in a very essential manner from 
those found among the rest of the primates—and all other verte- 
brates—in having its lower anterior border bent downward and for- 
ward so as toformachin. 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 figs. 77-81.) 
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 veitebrates has 
been curiously uniform from the very earliest times, as may be seen 
in the illustrations of mesozoic and eocene jaws. (See figs. 1-3.) 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 more interesting. 
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. 
The elephant (see fig. 8) has a kind of chin, and among older writers 
in the preevolutionary days this fact was adduced as showing its 
superiority to other quadrupeds. But we now know that the ele- 
phant’s chin is a mere degenerate remnant of the long lower jaw of 
his ancestors, the tetrabelodon (see fig. 6) and the mastodon (see 
fig. 7). 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 rela- 
tions. (See figs. 59-61.) Here a little search into paleontology shows 
that this apparent chin is not, like the elephant’s, a relic of decayed 
functions, but that it has, like thac of man, increased and improved 
with the ages. As seen in the illustrations, the dugong’s collateral 
ancestor, the halitherium, and its big extinct relative, known as Stel- 
1 Reprinted by permission from “‘ Knowledge,’’ London, November, 1913. 
2 Theillustrations are by Ménie Gowland. 
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