348 Matatioih in Mlcro-Orgavisms 



genetic constitution can tlierefoie hardly be sought — for the moment — 

 anywhere but in the organisms themselves. In Wolf's experiments, on 

 the other hand, there appears to be evidence that the variations 

 depended in some way upon the environment ; for they occurred most 

 frequently in organisms subjected to the action of poisons. But the 

 relation between the mutation and the chemical is not apparent. The 

 same substance seems to be able to produce two opposite results ; and 

 two different substances seem capable of producing the same result. 



The remarkable phenomena so well studied by Massini, Burri, and 

 others seem at first sight more illuminating. It appears that certain 

 bacteria, which cannot ferment a certain substance, can acquire the 

 power of fermenting it if kept in contact with it for a sufficient time. 

 At first the organisms cannot avail themselves of the new food around 

 them, but they then undergo a change which enables them to do so. 

 It seems at first sight that the new power is the result of necessity — 

 the " mutation " being a direct and indispensable adaptation to a 

 definite end. But to necessity alone the change can hardly be ascribed 

 — even by a confirmed Aristotelian. For it is apparent (e.g. from the 

 work of Massini) that the ability to attack a certain substance (in this 

 case lactose) is not a necessary condition for the survival of the race. 

 It is, rather, a luxury. The lactose-splitting individuals arise and 

 flourish in a non-lactose-splitting colony ; but the latter can survive for 

 a very long period, and there is nothing to prove that the new race 

 would supplant the old as a result of natural selection. If every 

 individual — as Burri supposes — possesses the power {in posse or in esse) 

 of fermenting the sugar ; and if, under ordinary cultural conditions, 

 only a minority avails itself of this power : then surely it seems absui'd 

 to suppose that the splitting of the sugar is necessary for the survival 

 of the race. These considerations do not affect the fact, however, 

 which seems to be established that there is a direct relation of some 

 sort between the sugar and the change which it produces in the 

 organism. The action of the sugar is specific. Lactose, and lactose 

 alone, makes B. coli mutabile able to ferment lactose, but does not 

 enable it to ferment saccharose or any other sugar. We are not 

 dealing here with a stimulus which may produce one of two opposite 

 reactions, or with a reaction which may be produced by another 

 stimulus. 



Variations of this sort seem to stand in a class by themselves. 

 Pringsheim (1910) calls them "functional adaptations" or "accom- 

 modations." It has been maintained that all variations in Protista are 



