132 FOOT-NOTES TO EVOLUTION. 



ample, does not give to the grown man his characteris- 

 tics. It gives only the power to acquire them. Just as 

 excessive muscular development requires excessive use 

 of the arm, so average development of any organ is con- 

 ditioned on an average degree of normal activity. 



"Therefore," says Mr. Reid, "// is clear that the full 

 development of the normal adult arm as well as many other 

 important structures is acquired, differing in this from 

 eyes, ears, teeth, nails, etc., which are wholly inborn 

 and do not owe their development in the least to use 

 and exercise. It will be found that adult man differs 

 physically from the infant almost wholly in characters 

 which are acquired, not in those which are inborn. In 

 teeth, hair, skull bones, and some other respects he dif- 

 fers from the infant as regards inborn characters, but as 

 regards almost all of the structures of the trunk and 

 limbs and most of those of the head, the difference is in 

 characters which have been acquired by the adult as a 

 response to the stimulation of exercise and use. . . . 

 But variations acquired as a result of use and disuse are 

 plainly never transmitted. Thus an infant's limb never 

 attains the adult standard except in response to the 

 same stimulation (exercise) as that which developed the 

 parent's limb. The same is true of all the other struc- 

 tures which in the parent underwent development as a 

 result of use or subsequent retrogression in the absence 

 of it. These, like the limbs, do not develop or retro- 

 gress in the infant except as a result of similar causes. 

 Plainly, then, what is transmitted to the infant is not the 

 modification, but only \.\i& power of acquiring it under simi- 

 lar circumstances, a power which has undergone such an 

 evolution in high animal organisms that, as I say in man, 

 for instance, almost all the development changes which 

 occur between infancy and manhood are attributable to it. 



" The power of acquiring fit modifications in response 



a 



