xx THE INFLUENCE OF ENVIRONMENT 385 



laborious research are indispensable, or we find ourselves 

 wrecked on insurmountable difficulties." 



What may be the outcome of further experiments on 

 the effects of use and disuse no one can tell, but as things 

 stand at present there are few adequately precise data 

 in regard to individual modifications due to peculiarities 

 in function, and no quite secure data warranting us in 

 believing that individually acquired functional modifi- 

 cations can be transmitted to the offspring as such or 

 in any representative degree. 



In this connection misunderstandings are apt to arise ; 

 thus reference is often made to cases like that of the 

 trotting horse. In 1796 its utmost speed was stated at 

 a mile in 2 min. 37 sec. ; in 1896 at 2 min. 10 sec. Does 

 it not follow that the trotting horse has been improved 

 by the transmission of the results of systematic training 

 in trotting ? It certainly does not follow from the 

 evidence available, which points to the conclusion that 

 the improvement has been due to the selective breeding 

 of the constitutionally swift. 



It is sometimes said that years of industry in an occu- 

 pation like shoemaking result in definite anatomical 

 changes in joints and muscles, and that evidence of the 

 entailment of these modifications may be detected in 

 men whose fathers and grandfathers were shoemakers. 

 If a case like this could be established beyond all criticism 

 it would be of great interest, but no perfectly certain 

 case is known. Where the offspring follow the same 

 occupation as their parents did, the same functional 

 modifications are of course reacquired. But this proves 

 nothing as to transmission. 



From one point of view the living body is a great 

 system of correlated chemical processes. Many different 

 lines of chemical change or metabolism go on simul- 

 taneously, but on the whole harmoniously. When there 

 is some marked peculiarity of function there is a local 

 predominance of a corresponding metabolism. Thus the 

 strengthening of muscle in a professional dancer's legs 

 implies an exaggeration of that line of metabolism (myo- 

 genic) which leads to the production of muscle-substance. 



26 



