52 THE DESCENT OF MAN. 



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 to th^ir 

 habit of fighting with their incisor teeth and hoofs. 



In the adult male anthropomorphous apes, as Rtitimeyer,'' and 

 others, have insisted, it is the effect on the skull of the great de- 

 velopment of the jaw-muscles that causes it to differ so greatly in 

 many respects from that of man, and has given to these anima's 

 "a truly frightful physiognomy." Therefore, as the jaws and 

 teeth in man's progenitors gradually became reduced in size, the 

 adult skull would have come to resemble more and more that of 

 existing man. As we shall hereafter see, a great reduction of the 

 canine teeth in the males would almost certainly affect the teeth 

 of the females through inheritance. 



As the various mental faculties gradually developed themselves 

 the brain would almost certainly become larger. No one, I pre- 

 sume, doubts that the large proportion which the size of man's 

 brain bears to his body, compared to the same proportion in the 

 gorilla or orang, is closely connected with his higher mental pow- 

 ers. We meet with closely analogous facts with insects, for in 

 ants the cerebral ganglia are of extraordinary dimensions, and in 

 all the Hymenoptera these ganglia are many times larger than in 

 the less intelligent orders, such as beetles.'' On the other hand, 

 no one supposes that the intelle'ct of any two animals or of any two 

 men can be accurately gauged by the cubic contents of their skulls. 

 It is certain that there may be extraordinary mental activity 

 with an extremely small absolute mass of nervous matter: thus 

 the wonderfully diversified instincts, mental powers, and affec- 

 tions of ants are notorious, yet their cerebral ganglia are not so 

 large as the quarter of a small pin's head. Under this point of 

 view, the brain of an ant is one of the most marvelous atoms of 

 matter in the world, perhaps more so than the brain of a man. 



The belief that there exists in man some close relation between 

 the size of the brain and the development of the intellectual facul- 

 ties is supported by the comparison of the skulls of savage and civ- 

 ilized races, of ancient and modern people, and by the analogy of 

 the whole vertebrate series. Dr. J. Barnard Davis has proved,™ 

 by many careful measurements, that the mean internal capacity 

 of the skull in Europeans is 92.3 cubic inches; in Americans 87.5; 



" 'Die Grenzen der Thierwelt, eine Betrachtung zu Darwin's Lehve.' 

 1S68, s. 61. 



™ Dujardin, 'Annales des So. Nat.,' 3rd series Zaolog. torn. xiv. 1850, 

 p. 203. See also Mr. Lowne, 'Anatomy and Phys. of the Musca voml- 

 toria,' 1870, p. 14. My son, Mr. P. Darwin, dissected for me the cerebral 

 ganglia of the Formica rufa. 



"" 'Philosophical Transactions,' 1S69, p. 513. 



