n.] ON HEREDITY. 85 



tendencies of the organ in question. We cannot by excessive 

 feeding make a giant out of the germ destined to form a dwarf; 

 we cannot, by means of exercise, transform the muscles of an 

 individual destined to be feeble into those of a Hercules, or the 

 brain of a predestined fool into that of a Leibnitz or a Kant, by 

 means of much thinking. With the same amount of exercise 

 the organ which is destined to be strong, will attain a higher 

 degree of functional activity than one that is destined to be 

 weak. Hence natural selection, in destroying the least fitted 

 individuals, destroys those which from the germ were feebly 

 disposed. Thus the result of exercise during the individual 

 life does not acquire so much importance, for, as compared 

 with differences in predisposition, the amount of exercise 

 undergone by all the individuals of a species becomes relatively 

 uniform. The increase of an organ in the course of generations 

 does not depend upon the summation of the exercise taken 

 during single lives, but upon the summation of more favourable 

 predispositions in the germs. 



In criticizing these arguments, it may be questioned whether 

 the single individuals of a species which is undergoing modi- 

 fication do, as a matter of fact, exercise themselves in the same 

 manner and to the same extent. But the consideration of a 

 definite example clearly shows that this must be the case. 

 When the wild duck became domesticated, and lived in a farm- 

 yard, all the individuals were compelled to walk and stand 

 more than they had done previously, and the muscles of the 

 legs were used to a correspondingly greater degree. The 

 same thing happens in the wild state, when any change in the 

 conditions of life compels an organ to be more largel}'- used. 

 No individual will be able to entirely avoid this extra use, and 

 each will endeavour to accommodate itself to the new con- 

 ditions according to its power. The amount of this power 

 depends upon the predisposition of the germ ; and natural 

 selection, while it apparently decides betw^een individuals of 

 various degrees of strength, is in truth operating upon the 

 stronger and weaker germs. 



But the very conclusions which have been drawn from the 

 increase of activity which has arisen from exercise, must also 

 be drawn from the instances of atrophy or degeneration fol- 

 lowing from the disuse of organs. 



