STORY OF THE CHIN—ROBINSON. 607 
of the jaw is already being dropped into something resembling a 
chin, and the depression at once becomes less apparent. Next are 
some jawbones of prehistoric man, namely, the Heidelberg and the 
Naulette jaws, in which the depression is still plainly seen and is 
scarcely less marked than in the gibbon. 
It will be seen that the Heidelberg jaw shows on its surface a 
tubercle; indeed, I understand that one of the descriptions of it 
published soon after it was found stated that it did not differ from 
modern jaws in this respect. (See fig. 41.) A brief comparison with 
the other casts, however, will make it plain that the tubercle here 
seen is too low down to be that for the genio-glossus, and is plainly 
the one for the genio-hyoid muscle mentioned in the earlier part of 
this article, which has nothing whatever to do with the tongue. 
This tubercle is quite common among the apes. 
When we come to the Pygmies and Bushmen we find in the 
majority of jaws the remains of this pit or a mere flat surface; but 
in some African dwarf races, and among the Hottentots, Veddas, 
and Andamanese, two little prominences are seen beginning to grow 
at the lower edge of the pit. (See figs. 19-29.) These tubercles, 
as we pass to higher and more civilized races, become more and more 
prominent, until we get the European type familiar to all students 
of anatomy. 
Now the bearing of these changes on the functions of the genio- 
glossus muscle is fairly evident. First of all, it needed a deep pit in 
the lower apes to get room to work at all. Then the depth of the 
pit became unnecessary through the tilting of the lower surface of 
the mandible; and by means of this change the muscle was obviously 
given greater freedom for action. Then we get a nearly flat surface; 
and finally a prominence appears, enabling the separate fasciculi of 
the muscle to spread from the very point of origin and so act inde- 
pendently without hampering their neighbors. (See figs. 18, 32,38, 42.) 
We are thus able to follow the whole course of the history of the 
genio-glossus muscle from fossil lemurs to modern men, and a very 
remarkable history it is; difficult, I believe, to parallel in any other 
structure of the body which we may pick out for the purpose. We 
found it in the lower apes, in which it first appears as an important 
factor in tongue movements, coming out of a hole in the lower jaw, 
and we take leave of it mounted upon a pinnacle quite as high as 
the pit was deep. (See figs. 63, 64.) This is as if an organism com- 
menced its career in the uttermost depths of the sea, and attained 
its full development at the top of Mount Everest! The muscle might 
stand above all things else in our bodies as a symbol and sign of our 
upward progress. For I think it can not be denied that its develop- 
ment marched part passu with the development of intellectual 
capacities and the increasing need of a means of clear expression. 
