122 ACQUIRED CHARACTERS sec. 



form of Prorsa it cannot revert to Prorsa, and therefore the 

 latter is not produced artificially. AVeismann further argues 

 that the different Prorsa generations must have different 

 physical constitutions, because the later of the two cannot be 

 converted by warmth into Prorsa. 



In answer to this, it is to be remarked that it was this 

 second generation of Prorsa which in one case on the applica- 

 tion of warmth again produced some Prorsa (Weismann's 

 experiment, 10 A). And thus the hypothesis of reversion 

 breaks down and that of the direct influence of warmth is 

 proved. 



Dorfmeister, however, has, as will be discussed in the fol- 

 lowing, by the application of warmth produced Prorsa out 

 of Prorsa. 



If warmth had less influence than cold there mioht be an 

 explanation for it. Weismann goes on the assumption that 

 artificially applied warmth affords the same stimulus as the 

 natural. I am of opinion that this is not to be expected. At 

 least in artificial experiments the effect of the light of 

 summer is wanting. Further, the duration of the period of 

 development in them has always been longer than in the case 

 of the warm forms under natural conditions. Besides, the 

 natural rise and fall of temperature, and in general the 

 naturally effective degrees of warmth as well as the effect of 

 radiant heat, cannot be imitated. So that the stimulus of 

 artificial warmth is not the same as that of natural. In the 

 case of cold the conditions are much simpler. 



In actual fact, however, Dorfmeister's experiments show 

 that artificial warmth has the same effect as natural. 



Taking all in all, from the facts already detailed, in my 

 opinion, the simple conclusion follows, that it is obviously 

 the action of warmth which in the natural state originally 

 produced one form of butterfly out of another, and that adap- 

 tation had nothinfy to do with the matter. 



