110 THE ENTOMOLOGIST. 



butterfly in the germ of the individual, I should no longer have 

 fallen into this confusion. Then I pictured to myself a cycle in 

 such a way, that one and the same germ-substance was so 

 governed, that it must one time produce levana, and the second 

 time prorsa, then again levana and again prorsa. To-day I 

 imagine two kinds of rudiments in the germ near one another, 

 one of which is enabled to develope by heat and the other 

 by cold. Now it is no longer a difficulty, that, according to 

 circumstances, from one brood of prorsa yet another brood of 

 prorsa follows, as I already had shown, that by the influence of 

 cold upon the pupae, one brood of levana could be followed 

 by a second brood of levana. In my view, the conception of 

 *' reversion " no longer in general plays any part in these 

 phenomena, but only that of the activity of one rudiment or 

 the other. 



The facts harmonise very well with this view of cyclic heredity, 

 although they also show, that the phenomena are not quite so 

 simple, as might be expected from it. This depends upon the 

 fact, that the temperature is not the only excitant, but that 

 rather some other co-operates with it : the tendency to alter- 

 nation. 



For the rest, I had already been able, from my old experi- 

 ments with levana, to draw the conclusion that the change of 

 form is a relatively free one, as in one of the same (Experi- 

 ment 6) I succeeded in getting a female of the summer form 

 prorsa to reproduce, and indeed, in the hot summer of 1869, 

 very early indeed — on July 4th. The butterflies (18 specimens) 

 developed from the eggs after 30-31 days, and these were all the 

 prorsa-iorm. One of my critics rightly cited this against me at 

 the time. 



It is obvious, from the experiments now set forth, that in 

 fact the third broods can be induced by heat to assume the 

 prorsa-ioim., at least in part ; indeed, that an especially high 

 temperature is not always necessary for isolated prorsa-forms to 

 arise. This is proved by : — 



The first experiment, in which, from about 60 pupae of the 

 third brood, which had pupated at the beginning of September 

 at the usual temperature of a warm room, one prorsa, at least, 

 still developed. 



The third experiment, in which, from numerous specimens, 

 which pupated in August (consequently very early), 15 prorsa 

 butterflies emerged up to the end of August. 



The fourth experiment, in which, from 241 pupae of the third 

 brood, five prorsa emerged at the end of August at the usual room 

 temperature. 



The fifth experiment, in which, from pupae of the third brood, 

 56 prorsa butterflies emerged at the incubator temperature of 

 27-28° C. at the beginning of September. 



