iv ARTIFICIALLY PRODUCED FORMS 133 



the Levana. In the case in which he got the darkest and 

 the simplest Prorsa he applied a still higher temperature than 

 Weismann, although only for a short time : the caterpillars, as 

 soon as they had hung themselves up to change into the 

 pupa stage, "were exposed in the morning on a part of the stove 

 to a maximum temperature of 32. 5 C. and towards midday 

 had become pupae ; " they afterwards developed in the room. 



Since the pupa, which under natural conditions produces 

 Prorsa, has a dormant period of only a few days, while that 

 which produces Levana has one of six months, Dorfmeister, by 

 increase of temperature and the consequent abbreviation of 

 the period of development, has simply imitated the effects of 

 summer. 



But the fact which appears to me of the utmost importance 

 is that Dorfmeister, from different temperatures and different 

 periods of development, has obtained a whole series of 

 stages of modification between Levana and Prorsa, while such 

 intermediate stages under natural conditions very rarely 

 occur. He himself remarks that during more than forty years' 

 collecting he met with only one specimen of such an inter- 

 mediate stage in the wild state, in places where Levana and 

 Prorsa were quite common. The rarity of the intermediate 

 forms (called Porima) is evidently the reason that Levana and 

 Prorsa have been considered distinct species. Under natural 

 conditions, therefore, there is obviously as a rule only one 

 average of summer temperature which produces the ordinary 

 Prorsa. But Dorfmeister has by the application of tempera- 

 tures beneath this average in a few experiments artificially 

 produced a whole series of such transition stages. These 

 stages can be divided into two groups (1) those which must 

 be described as transition forms between Levana and Prorsa ; 

 (2) those which are different stages of Prorsa. Thus, while 

 the latter were produced by the highest temperature and 

 the shortest period of development, the former were reared 



