52 THE NATURE AND ORIGIN OF LIFE 



reactions, the living being decreases and finishes by disap- 

 pearing. 



We are accustomed to say that a being is alive as long as 

 assimilative reactions, no matter how slight, go on in it. Now 

 what we have been saying proves at once that observation 

 will often be unable to establish directly any visible growth. 

 Growth can be verified only during the individual's period 

 of growth ; it disappears completely in the adult state and 

 its place is taken by diminution during the period of decay 

 which leads to death. 



Thus, during the period of growth, assimilation is an 

 approximate law tempered in its scientific rigour by destruc- 

 tive reactions ; in the adult state the corrections to be made 

 in the law are as important as the law itself ; and they be- 

 come more important all through the period of decrepitude 

 which leads to death. To try to find in this last period an 

 example of the law of assimilation would be the same thing 

 as to apply Marriotte's Law to a saturated vapour. 



In certain cases it has been possible to separate experiment- 

 ally the phenomena of assimilation from those phenomena of 

 destruction which are regularly superposed in nature. This 

 was done, for example, in the case of the bacteridia of an- 

 thrax ; it is a fact to be remembered, as it helps us to an 

 important idea. 



1. At 35C., in a proper culture-medium, the bacteridia of 

 anthrax multiply without any modification. This is an 

 example of the rigorous application of the law of assimila- 

 tion. 



2. If you add to the culture-medium a small quantity 

 of phenic acid or permanganate of potash, the bacteridia 

 will still multiply but more slowly, and they undergo certain 

 changes ; a part of these we are able to study under the 

 name of attenuation of virulence. This is an example of 



