126 THE NATURE AND ORIGIN OF LIFE 



increasing virulence rather than in that of decreasing virulence 

 (this, of course, is the Darwinian thesis). But among the 

 bacteridia thus multiplying all those which chance to have 

 undergone variation in the sense of diminished virulence 

 are stopped short by the sieve, mouse, which destroys them, 

 whereas those which, equally by chance, have undergone 

 variations in the sense of increased virulence pass through 

 the sieve mouse and survive. And so virulence increases in 

 proportion as generations of bacteridia pass through the 

 mice, that is, on meeting the same sieve which keeps on 

 realizing the same selection. 



The result increased virulence is therefore the same 

 as in the Lamarckian thesis for all surviving descendants of 

 the original bacteridia. In the experiment which we have 

 described there is nothing to make us choose one interpreta- 

 tion rather than the other, since in both we only verify the 

 final result of things. 



In the case of the bacteridia a very great number of 

 individuals have been produced without our being able to 

 tell the number of those which have died as a consequence 

 of being ill-adapted. And so we have no answer to give to 

 Darwinians when they assert, on the faith of their system of 

 interpretation, that many bacteridia chance to be ill-fitted 

 for the struggle and succumb in the course of our experi- 

 ments. 



But the case is not the same when, instead of the bacteri- 

 dia, we make our observations on the sheep which is their 

 host. Suppose, in fact, that, quite contrary to what has 

 passed in the previous experiment, the inoculated sheep 

 should resist the anthrax. It must have digested and 

 assimilated anthrax bacteridia, which proves by definition 

 that the bacteridia with which it was inoculated were not 

 virulent in its case. But this sheep which has been cured of a 



