264. THE THEORIES OF EVOLUTION 



gone are absolutely fortuitous as far as the environ- 

 ment goes.) 



"The sheep will in this case play the part of a sieve. 

 Those of the bacilli which, after the variations have 

 taken place, happen to be virulent for sheep, that is 

 fit to survive in sheep, will develop in the animal's 

 internal environment. On the other hand those which, 

 after the variations have taken place, do not happen to 

 be virulent for sheep, will die off in the animal's in- 

 terior environment, for according to the definition of 

 non-virulence, they are not fit to multiply in this inter- 

 nal environment. In the end the virulent bacilli 

 will kill the sheep and will remain alone in its blood; 

 thus the fittest will survive as a consequence of the 

 selection operated by the body of the sheep." 9 



Let us see now how these bacteria go through a 

 process of adaptive evolution. A well-known experi- 

 ment instituted by Pasteur, Chamberland and Roux has 

 demonstrated that if anthrax bacillus not virulent 

 enough to kill a sheep is inoculated in a mouse one 

 day old it will kill it and then become virulent enough 

 to kill a mouse one week old; it can then kill succes- 

 sively an adult mouse, then a guinea-pig and finally 

 a sheep. A Darwinian would relate the experiment 

 as follows: 



"The bacillus passing through the newly-born, the 

 older and the adult mouse, encounters continuously, at 



s Elements de Philosophie Biologique, pp. 136-137. 



