76 THE NATURE AND ORIGIN OF LIFE 



not concern us here ; and moreover the intervention of A 

 counts for very little in B's evolution. 



However all this may be, the two factors A and B inter- 

 vene every instant to modify each other reciprocally ; and 

 the phenomenon which brings them together the life of A 

 must be considered as the struggle between these two factors. 



We shall find this a handy expression l in describing 

 experiments in which we vary at will some measurable part 

 of the sum total B. We say that we introduce a new enemy, 

 or suppress a pre-existing enemy, in the struggle sustained 

 by A against exterior circumstances. 



So long as A remains alive, we declare that it triumphs in 

 the struggle, even though it undergoes modification ; and it 

 is these modifications which we have to determine. The 

 study of them will give us our great biological law. 



It may seem odd that we should give this same name of 

 enemy to substances which, like oxygen or food, are indis- 

 pensable to the conservation of life in the individual, and 

 to substances like poison, toxins, venoms, whose ordinary 

 result is to induce death. But the interesting part of our 

 researches is precisely to verify the generality of results 

 obtained and to enable us to gather up into a single formula 

 conclusions relating to the struggle with foods or against 

 poisons, provided the individual survives. This will also 

 enable us to extend the law which w r e discover to all varia- 

 tions of the factor B, even when such variations are too 

 complex to be subjected to experiment. 



1 This expression has already been employed in certain eases ; 

 we say, for example, that such a condition of the environment 

 provokes such a reaction of a certain organism. 



