METHODS 51 



destruction of the bacterium we are studying. This means 

 that, in certain conditions properly chosen by a skilful ex- 

 perimenter, the bacterium by assimilation shows its quality 

 as a living being ; in other conditions, on the contrary, 

 it behaves like ordinary chemical bodies and is destroyed 

 while reacting. This destruction which the bacterium under- 

 goes may, moreover, be brought about in any number of 

 ways, according to the reagents used and the conditions in 

 which they are used. 



Finally, it must be further observed that destructive 

 reactions are necessarily much easier to bring about than 

 the constructive reaction ; if you are given haphazard a 

 bacterium and a liquid, and if you plunge the bacterium 

 into the liquid, you are very much more likely to see it die 

 than to see it live and prosper. Assimilation is the excep- 

 tion, destruction is the rule. A living body given up to change 

 is much more likely to die than to live. No wonder that 

 when we place it in the natural conditions where it is 

 able to live, that is, to grow and multiply, it finds along 

 with favourable conditions other harmful circumstances, 

 save in the altogether exceptional case where I suppose the 

 bacterium multiplying without variation. 



Here we find with no great difficulty the cause of the mis- 

 take which generally makes the law of assimilation a merely 

 approximate law. Destructive reactions peculiar to the 

 living body under consideration, but not characteristic of it 

 as a living being, are found in nature superposed with its 

 assimilative reactions. 



When the assimilative reactions are stronger than the de- 

 structive, the living being grows. When they balance each 

 other exactly, the living being does not change perceptibly 

 in dimensions, and we say that it is adult. But when, finally, 

 the destructive reactions carry the day against assimilative 



