112 THE ENTOMOLOGIST. 



cannot produce the natural conditions artificially in the incubator; 

 the change between day and night temperature is not to be closely 

 imitated, and still less the wind, &c., or indeed the direct rays of 

 the sun, which also come into consideration. 



I consequently believe we may assume, that in fact the third 

 brood of levaiia-prorsa, as a general rule, has the tendency to 

 longer pupal rest {i. e. to hybernation), and with it to the levana- 

 form ; but that this is not the case with all the brood, single in- 

 dividuals in many broods being constantly present, which, on 

 the other hand, bear in themselves the tendency to rapid de- 

 velopment and to the assumption of the prorsa-foim. These are 

 the individuals which still produce prorsa butterflies at a medium 

 temperature (at about 13-18° C.) in September or October. But 

 manifestly we must conclude, that a larger number of pupae of 

 this third brood is altered by the higher temperature, and may 

 be induced to develop forthwith under the assumption of the 

 prorsa-iovm. Whether there are also such individuals, which 

 are in no way to be induced to this, must for the present remain 

 undecided ; the five porima specimens of the experiment of 1869 

 prove that with many it only takes place with great difiiculty, 

 as in these both germ-rudiments (Keimesanlagen), the prorsa 

 and the levana rudiment, have worked together. 



But such a co-operation can, as the experiments teach us, 

 take place in yet another way. In the first place it is significant, 

 that specimens oi porima sometimes occur also in the open, and, 

 indeed, in the summer. If I am not mistaken, they can be 

 brought about in two ways : either as in the experiment of 1869, 

 i. e. by the fact that an unusually hot summer allowed the third 

 brood to begin very early (commencement of July), so that their 

 pupge fell under the influence of the greatest summer heat. In 

 this case those individuals will become porima, in which the 

 tendency to levana is overcome by the heat with most difficulty.. 

 But the porima-ioYm. can, indeed, arise in yet another way — in 

 which they were first artificially produced by Dorfmeister, and 

 later on by myself, namely, by the action of a lower tempera- 

 ture on the second brood. In this a preponderance of the ten- 

 dency to the ^7'o?-S6/ -form will have to be assumed, but one which 

 can be more or less completely overcome by the fact that a low 

 temperature is acting at the critical time, i. e. immediately after 

 pupation. 



It seems obvious from the experiments, that a certain in- 

 fluence can still be always exercised by temperature on the 

 colouring of the butterfly, even in the later period of the develop- 

 ment of the pupa. Experiment 2 shows, at least, that pupae 

 of the third brood of the end of August and beginning of 

 September, which not only spent the winter in the cold, but 

 were also prevented from further development the whole of the 

 spring by means of cold, produced, indeed, the levana-ioxm. in 



