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Show 144 THE DESCENT OF 1\'LA.N. PART I. habitual use, this shews that certa~n 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 arms and hands, partly the cause and partly the result of man's erect position,. app~ars to have led in an indirect manner to other modtficatwns of structure. The early male progenitors ~f man were,. as previously stated, probably furnis~ed w1th gr~at can~ne t th . but as they O'radua11y acqmred the habit of usmg ee ' b . . h th . stones clubs or other weapons, for fightmg Wit elF enemi~s, the~ would have used their jaws and te~th less and less. In this case, the jaws, together w1th 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 canme 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 Riitimeyer, 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 Betrachtung zu Darwin's Lehre,' 1868, s. 51. -cu.AP. IV. MANNER OF DEVELOPMEN l'. 145 man. A great reduction of the canine teeth in the males would almost certainly, as we shall hereafter see, have affected through inheritance the teeth of the females. As the various mental faculties we1·e gradually developed, the brain would almost certainly have become larger. No one, I presume, doubts that the large size of the brain in man, relatively to his body, in compari• SOn with that of the gorilla or orang, is closely connected with his higher mental powers. We meet with closely analogous facts with insects, in which the cerebral gauglia are of extraordinary dimensions in ants; these ganglia in all the Hymenoptera being many times larger than in the less intelligent orders, such as beetles.69 ·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 generally known, yet their cerebral ganglia are not so large as the ·quarter of a small pin's head. Under this latter point of view, the brain of an ant is one of the most marvellous atoms of matter in the world, perhaps more marvellous .than the brain of 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 civilised races, of ancient and modern people, and by the analogy of the whole verte- 69 Dujardin, 'Annales des Sc. Nat.' 3rd series, Zoolog. tom. xiv. 1850, p. 203. See also Mr. Lowne, 'Anatomy and Phys. of the Musca .vomitoria,' 1870, p. 14. My son, Mr. F. Darwin, dissected for me the <!erebral ganglia of the Formica ruja. VOL. I. L |
