RELATIONS OF SPEECH TO HUMAN PROGRESS 531 



of the under-jaw, and hence the characteristic chin which so 

 distinguishes the human countenance. 



By this radical change of structure (for it involved a complete 

 departure from the fixed type of mandible common among all 

 vertebrates) the muscle was at once set free and the separate 

 fasciculi were enabled to act upon the under-surface of the 

 tongue without being hampered by overcrowding. Even now 

 mechanical perfection was not quite reached, for it is obvious 

 that if the fan-like muscle sprang from a prominence the requisite 

 independence of its component parts would be facilitated still 

 more. This would necessitate still further room in what was 

 originally the cramped space between the tongue and the inferior 

 maxilla, which could only be obtained by a still further tilting 

 forward of the lower margin of the bone. 



Now when we come to examine by comparative methods the 

 jawbones of apes, prehistoric men, primitive savages with im- 

 perfect articulate speech, and finally the more highly developed 

 and civilised races the world over, we find indubitable evidence 

 of such changes having taken place. The writer for many years 

 has been collecting specimens or making plaster casts of this 

 part of the jawbone, and a mere glance at the complete series 

 demonstrates the facts with scarcely any further explanation. 

 First there is the usual type of monkey's jaw with its deep pit, 

 sometimes almost penetrating through to the anterior surface. 

 Then among certain anthropoids where a decided tilted move- 

 ment has begun, such as the chimpanzee and certain of the 

 gibbons, the pit becomes shallower because it was no longer 

 so much needed. In certain prehistoric jaws such as the 

 Heidelberg and Naulette specimens the pit is still there, but 

 has become shallower still. Among practically all the Bush- 

 men, and many of the Central African Pygmies, Andamanese 

 and Veddahs, there are still signs of the pit, but on the whole 

 the surface is a flat one with only slight roughnesses upon it. 

 In several interesting specimens of Hottentot jaws the genio- 

 glossus tubercles are seen as tiny prominences coming up from 

 the lower side of the cavity, while in practically all the peoples 

 of the earth who have adapted an elaborate form of articulate 

 speech the whole inner surface of the jaw from above down- 

 wards is slightly convex, and in the centre of it are the genial 

 prominences that are described in all current works on anatomy. 



An examination of the development of this part in the young 



