No. 592] EVOLUTION OF THE CHIN 



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it. The next question is: if the jaw is in this way being 

 reduced, should we naturally expect it to be equally re- 

 duced in all directions ? There are reasons why we might 

 anticipate that it would not. 



If we consider especially the horizontal ramus of the 

 jaw, the fact is striking that not all parts of it are equally 

 permanent. The teeth themselves, and the upper border 

 of the ramus, are temporary structures. In old age, the 

 teeth are lost. The upper margin of the jaw itself is, in 

 late life, reabsorbed (Fig. 5), which, with a corresponding 

 loss in the upper jaw, produces the well-known nut- 

 cracker appearance of the aged human face. Without at- 

 tempting to dogmatize, I will go so far as to say that we 

 might confidently expect that the region in the lower jaw 

 which is lightest in structure, and the first to disappear in 

 the individual, would be the part which would naturally 

 respond first to the influence of external environment. 

 Put in another way, the suggestion might be worded thus : 

 "We recognize as the important thing in the jaw the teeth. 

 Hence, as smaller teeth became more appropriate through 

 change of habit and environment, changes would first ap- 

 pear in these teeth themselves, and in the tissues which 

 immediately support them. In fact, in the fossil " Hei- 

 delberg" jaw the teeth have been reduced faster than the 

 jaw itself. 3 Granted that our ape, ancestor had a jaw, it 



