448 THE ASCENT OF MAN. 



produce greater changes, and long-continued repetition notably alters 

 even the hardest and most enduring of structures. Thus it is that 

 bones are modified in form by muscular pull and the surfaces of teeth 

 are shaped by incessant grinding. These alterations are more readily 

 apparent to us because they affect very hard and easily preserved 

 organs, but the effects are equally potent, though not so clearly recog- 

 nizable, in the softer tissues of the body. Every act of our lives is cer- 

 tainly but surely registered within the marvellous structure of our 

 bodies. Not a muscle can contract without an absolute change sub- 

 stance ; in its not a nerve-cell can discharge with out some self-destruc- 

 tion. 



Most of these changes being very minute and evanescent are quite 

 beyond our power to accurately estimate, and were the increments of 

 change confined to a single life-time, were each individual to stand only 

 for himself and compelled to earn his experience by the same tedious 

 struggle, use and adaptation would have but little power to mold man- 

 kind into races and varieties. But, by the action of a law as yet im- 

 perfectly understood, the adaptations of each individual are transmitted 

 to its offspring ; or, to speak more accurately, the offspring pass through 

 the changes more easily and quickly than the parent did. While each 

 has always to go back to the beginning and commence from the simple 

 blastema of the primitive egg, the younger has the advantage of being 

 able to adapt itself more quickly to its surroundings, provided these 

 have not too greatly changed, and thus starts a little way ahead of its 

 ancestor in the race for life. In consequence of this law, changes be- 

 come cumulative, and a cause acting for a great length of time upon a 

 series of successive generations finally produces a well-marked and 

 easily observed effect in the structure of individuals; changing colors, 

 modifying organs, shaping whole regions of the body. 



Again, if after such changes have been effected, these causes cease 

 to operate and the organs they have shaped are no longer of use, the 

 latter become reduced in size, atrophy, and recede, remaining however 

 iu a vestigial condition for many, many generations as records of the 

 past history of the race, as dolmens and cromlechs certify to former 

 customs and flint arrow-heads and stone hatchets give evidence of a 

 previous state of civilization. 



The human body abounds in testimony of this sort, — indications of 

 the pathway by which humanity has climbed from darkness to light, 

 from bestiality to civilization, — relics of countless ages of struggle, 

 often fierce, bloody, and pitiless. 



These are found in every organ of the body, and each new investiga- 

 tion adds to their number. To enumerate them all would be impossible 

 within the limits assigned me by your patience. I will therefore touch 

 only ujx)!! a few of the more striking ones, especially those connected 

 with the ujodifications of the limbs, with the erect i)osition, and witli 

 tihe segmentation of the body, 



