METHODS 89 



lions when the sheep has died from the attack of the multi- 

 plied bactendia, that all these bacteridia have in the highest 

 degree precisely the aptitude to struggle against sheep. In 

 other words, that which multiplied in the sheep was bac- 

 teridian organs of struggle against the sheep. There was, 

 indeed, an assimilation of the sheep's substance by the bac- 

 teridia, but it was not any and every assimilation. The 

 bacteridium's assimilation in the sheep does not give the 

 same results as the same bacteridium's assimilation in phen- 

 icated bouillon ; the assimilation is relative to the function 

 performed or, more briefly, it is functional assimila- 

 tion. 



This is the great biological law : it is the literal transla- 

 tion of Lamarck's principle " The function creates the 

 organ." Indeed, the function defines the organ as we have 

 seen ; and if circumstances are such that the same func- 

 tion is exercised for a long time, the corresponding func- 

 tional assimilation transforms the given organism into the 

 organ itself of the function. 



We have noted that habit has limits ; beyond a certain 

 degree no individual can acquire habituation to given con- 

 ditions. This means that the parts of the mechanism which 

 are less useful in executing a function long repeated are not 

 on that account completely destroyed ; enough remains 

 for other functions to be possible. No individual ever 

 adapts itself so exclusively to one kind of life as to become 

 incapable of all others. 1 



An experiment of Pasteur, Chamberland, and Roux, 

 is of great interest to us in this connexion. 



1 There are, however, parasites very strictly adapted to their 

 hosts for example, the parasite of malaria in man's blood. But 

 even this parasite, delicate as it is in its needs, adapts itself to the 

 conditions found in the body of mosquitoes of the genus Anopheles, 



