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, “7¢ zs clear that the full 
development of the normal adult arm as well as many other 
important structures 1s 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 asa result of similar causes. 
Plainly, then, what is transmitted to the infant is not the 
modification, but only the 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 
