80 THE DESCENT OF MAN 



great development of tlie jaw-muscles that causes it to differ 

 so greatly in many respects from that of man, and has given 

 to these animals "a truly frightful physiognomy." There- 

 fore, 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 presume, 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 powers. "We meet with closely analo- 

 gous 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 intelli- 

 gent orders, such as beetles." On the other hand, no one 

 supposes that the intellect 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 affections of ants are notorious, yet their cere- 

 bral 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 marvellous 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 faculties is supported by the comparison of the 

 skulls of savage and civilized races, of ancient and modern 

 people, and by the analogy of the whole vertebrate series. 



's Dujardin, "Annales dea Sc. Nat.," 3d series, Zoolog., torn, xiv., 1850, 

 p. 203. See also Mr. Lowne, "Anatomy and Phys. of the Musca vomitoria," 

 1870, p. 14. My son, Mr. P. Darwin, dissected for me the cerebral ganglia of 

 the Formica rufa. 



