iv INTERPRETATION OF THE FACTS 125 



Weismann, regarding the reappearance of the cold variety 

 as a reversion, not recognising the direct influence of cold, but 

 rather postulating internal causes, attempts to discover other 

 causes which might likewise determine this reversion. And 

 he believes he can point out warmth and mechanical move- 

 ment as such additional causes. 



In the summer of 1869 great heat prevailed at the time of 

 the development of the second summer generation of Levana- 

 Prorsa, From a batch of this generation an unusually large 

 proportion of Porima were developed (out of sixty to seventy, 

 eight to ten.) 



As Porima is half way to the cold variety, the fact is to 

 me very incomprehensible, and I should suppose that other 

 causes than the extra warmth determined the formation of 

 Porima, for to assume this as the cause would be to com- 

 pletely contradict all the facts previously detailed. 



In another case, a number of Pieris Napi, although they 

 had been reared in a heated room, all emerged in the following 

 spring as the winter form. But the pupae had made in the 

 summer a seven hours' railway journey ; and so the shaking 

 on the railway is taken as the cause of the reversion in this case. 



Unable as I am to grant that causes so completely opposite 

 as warmth and cold have the same effect in the modification 

 of forms, I am equally unable to agree that such different 

 causes as warmth and mechanical shaking produce the same 

 result. Since warmth has obviously, under natural conditions, 

 determined the transformation into new forms in the cases 

 previously discussed, I shall hold for the present that only 

 cold has the power of causing a reversion to the original form. 

 That effect of warmth, under natural conditions, which con- 

 sists in the formation of new varieties which are so distinct 

 that they have been in many cases long regarded as species, 

 has been evidently a direct effect. And it would need, as I 

 before pointed out, only a slight change in the external con- 



