88 THE NATURE AND ORIGIN OF LIFE 



for sheep. 1 On the contrary, the bacteridia of the sheep, 

 if they triumph, acquire an increase of virulence. 



As we do not know the structure of a bacteridium, we 

 may imagine provisionally that it comprises different active 

 parts and that these parts arrange themselves differently 

 in case of a new struggle, so as to realize a mechanism fitted 

 to the struggle ; or again that, of the different constituent 

 parts of the bacteridium, some do more than others to 

 realize the mechanism made necessary by the sum total 

 B of exterior conditions. If it were not a fault in method, 

 we might compare the bacteridium, a simple being, with man, 

 a complex being. When man performs an act or exercises 

 some function determined by circumstances, the man's 

 different parts do not intervene equally in the execution 

 of the act. To fix our ideas, we may suppose that the same 

 thing takes place in a bacteridium A which the sum of 

 circumstances B leads to exercise the function (A x B). 



Without further reference to such hypotheses, we can 

 verify that, under the prolonged influence of a sum of cir- 

 cumstances determining in the bacteridia the function 

 (Bacteridia x Sheep), there appears in the conquered sheep 

 a race of bacteridia which have acquired exactly the char- 

 acter corresponding to the execution of the function, namely, 

 resistance to the sheep. 



We have named organ all those parts taken together which 

 work in union with each other in the exercise of a function. 

 The total function of the bacteridia enclosed within the sheep 

 is determined by the nature of the sheep. So we can con- 

 sider, at each successive instant, the actual mechanism of the 

 parasite bacteridium as the organ of the struggle against 

 the sheep. We also verify, after a certain number of genera- 



1 Also by the loss of the faculty of giving off spores. 



