34 A STUDY IN HEREDITY 



immunity is not transmissible, it becomes very 

 improbable that other acquirements, which merely 

 affect the mind (brain), or the lungs, or arms, or 

 legs, or any other local structure, are transmissible. 

 It is worth while to put the question clearly, even 

 at the cost of a little repetition, which will not 

 matter if clarity of conception be gained thereby 

 If, as is alleged by most medical men, the effects of 

 disease are transmissible, then their effects must 

 accumulate generation after generation. The son 

 must start with the parent's constitution plus the 

 effects of the parent's disease, the grandson must 

 start with the son's constitution plus the effects of 

 the son's disease, and so on. It is plain on this 

 hypothesis that a race afflicted by any disease should 

 undergo evolution or degeneration — evolution if the 

 disease tends to strengthen the individual against 

 subsequent attacks by conferring immunity, de- 

 generation if it tends to weaken him. On the other 

 hand, if the effects are not transmissible, then a race 

 afflicted by deadly disease would change equally, 

 would undergo evolution equally — but not degenera- 

 tion. For men differ individually in their inborn 

 power of resisting disease. Deadly disease is there- 

 fore a selective agency. It weeds out the less 

 resistant to it, leaving the race to the more resistant. 

 Therefore, on this other, this Darwinian hypothesis, 

 a race afflicted by a disease should grow more and 



