38 Studies in the Theory of Descent. 



that time to their emergence, on 28th June 3rd 

 July, great heat prevailed. Now, while the inter- 

 mediate form Porima had hitherto been a great 

 rarity, both in the free state and when bred, having 

 never obtained it myself, for example, out of many 

 hundreds of specimens, there were among the 

 sixty or seventy butterflies that emerged from the 

 above brood, some eight to ten examples of 

 Porima. This is certainly not an exact experi- 

 ment, but there seems to me a certain amount of 

 probability that the high summer temperature in 

 this case brought about reversion. 



Neither for the second cause to which I have 

 ascribed the power of producing reversion can I 

 produce any absolute evidence, since the experi- 

 mental solution of all these collateral questions 

 would demand an endless amount of time. I am 

 in possession of an observation, however, which 

 makes it appear probable to me that continuous 

 mechanical movement acts on the development of 

 the pupae in a similar manner to cold, that is, 

 retarding them, and at the same time producing 

 reversion. I had, in Freiburg, a large number of 

 pupae of the first summer brood of Pieris Napi, 

 bred from eggs. I changed residence while many 

 caterpillars were in course of transformation and 

 travelled with the pupae in this state seven hours 

 by rail. Although this brood of P. Napi, under 

 ordinary circumstances, always emerges in the 

 summer, generally in July of the same year, as the 



