22 A STUDY IN HEREDITY 



treated adequately here. Two arguments more 

 I may use, however, partly because they have not 

 been developed, to my knowledge, by other writers, 

 and partly because they seem to me well-nigh 

 decisive. The more than normal development of 

 the blacksmith's arm is rightly called an acquired 

 trait, since it arises from exercise, from use, not 

 from germinal conditions. But no infant's arm 

 develops into an ordinary adult arm without 

 exercise similar in kind to that which develops 

 the blacksmith's arm, though less in degree. 

 Without the exercise, as when paralysed, it re- 

 mains more or less infantile. Therefore, almost 

 all that separates the infantile from the adult arm 

 is acquired. The same is true of most of the other 

 structures of the body, which do not develop except 

 under the stimulation of use. Thus brain, heart, 

 lungs, legs, all develop in this manner. Man's 

 physical acquirements are therefore vast. When 

 are any of them transmitted? Every infant has 

 to make afresh under similar stimulation the modi- 

 fications its parent so laboriously acquired. If it 

 be argued that exercise and use increase, not only 

 the individual's acquirements, but also his power 

 of making them, and that it is the latter that is 

 transmitted, I have only to reply that, in the 

 passage from infancy to old age, the power of 

 making acquirements constantly declines. In the 



