224 EXPERIMENTAL EVOLUTION LECT. 



taken for no other. But this bacillus, if you alter even 

 very slightly some of the external conditions it lives 

 in, gradually loses its most important character ; it is 

 deeply modified for the time being that is for the 

 time you compel it to live under particular conditions, 

 and perhaps even for some time the normal conditions 

 are restored. The usefulness or unusefulness of the 

 modification are not in the slightest degree apparent, 

 and to evolutionists it matters little, since modifications 

 may be useful, useless, or indifferent, or even injurious, 

 and natural selection destroys all that is not useful, or 

 at least indifferent ; but the main fact is that here is an 

 important modification which puts in its appearance 

 when some external conditions are changed, and dis- 

 appears when the normal condition reappears. Is envi- 

 ronment operative here, or must we assume that when 

 Bacillus anthracis is cultivated in some particular 

 manner, it loses its most important character without 

 any reference to the change of environment ? I dare 

 say no bacteriologist or physiologist could be met who 

 would venture to assert that the change of character 

 is not in direct relation with the change of external 

 conditions, whether the former is beneficial or not. 



It seems, then, that Prof. Weismann goes certainly 

 too far when he asserts that we have no proof of 

 the direct production of transmissible changes by 

 means of external influences. It may be said that he 



