144 
THE DESCENT OF MAN. 
Part I. 
habitual use, this shews that certain actions are 
habitually performed and must be serviceable. Hence 
the individuals which performed them best, would tend 
to survive in greater numbers. 
The free use of the anus and hands, partly the cause 
and partly the result of man’s erect position, appears to 
have led in an indirect manner to other modifications of 
structure. The early male progenitors of man were, as 
previously stated, probably furnished with great canine 
teeth : but as they gradually acquired the habit of using 
stones, clubs, or other weapons, for fighting with their 
enemies, they would have used their jaws and teeth less 
and less. In this case, the jaws, together with the 
teeth, would have become reduced in size, as we may 
feel sure from innumerable analogous cases. In a future 
chapter we shall meet with a closely-parallel case, in 
the reduction or complete disappearance of the canine 
teeth in male ruminants, apparently in relation with the 
development of their horns ; and in horses, in relation 
with their habit of fighting with their incisor teeth and 
hoofs. 
In the adult male anthropomorphous apes, as Kuti- 
meyer , 68 and others have insisted, it is precisely the effect 
which the jaw-muscles by their great development have 
produced on the skull, that causes it to differ so greatly 
in many respects from that of man, and has given to 
it “ a truly frightful physiognomy.” Therefore as the 
jaws and teeth in the progenitors of man gradually 
become reduced in size, the adult skull would have 
presented nearly the same characters which it offers in 
the young of the anthropomorphous apes, and would 
thus have come to resemble more nearly that of existing 
68 ‘Die Grenzen der Thierwelt, eine Betraelitung zu Darwin’s 
Lehre,’ 1868, s. 51. 
