METHODS 125 



Genuine natural selection is realized in the case where 

 the conditions B, in which the variations have been produced, 

 are at the same time the instrument of selection. In con- 

 ditions B, if we accept the Darwinian doctrine, the de- 

 scendants Aj, A 2 , A 3 , . . ., of a living body A undergo varia- 

 tions which, no matter of what kind they be, are evidently 

 due to the reactions produced by the medium B and yet have 

 no direct relation with fitness to survive in such conditions 

 B. Among these descendants some chance to be unfitted 

 to survive in conditions B and consequently die. Others, 

 on the contrary, chance to be fit to survive in such con- 

 ditions and multiply in their own likeness so long as the 

 conditions are not changed. 



Thus the result is the same as in the Lamarckian doctrine 

 since the only individuals that remain alive are adapted to 

 the conditions in which they live. 



Anthrax bacteridium gives us an example of such genuine 

 natural selection in a celebrated experimental case. In the 

 experiment of Pasteur, Chamberland and Roux, already 

 narrated (page 89) in Lamarckian language, a bacteridium 

 had been deprived of virulence for sheep or, at least, had too 

 little to flourish directly in a sheep, but was still capable of 

 living in a new-born mouse and was able to kill it. It was 

 passed on to an older mouse, and then to an adult, and so 

 on, and its descendants in the long run were again able to 

 kill sheep. Now, in Darwinian language, this experiment is 

 told as follows : 



The bacteridium, as it passes through the mice, new-born, 

 older, adult, everywhere finds the same sieve mouse a sieve 

 that is finer and more precise in proportion as it is an older 

 mouse. The bacteridium multiplies under these conditions, 

 undergoing variations without order and in every direction 

 variations which have no reason to be directed in the sense of 



