SEASONAL DIMORPHISM OF LEPIDOPTERA. 243 



brooded butterflies, occasionally saw the whole brood remain 

 latent without any special influences ; so it must here also be the 

 result of internal causes. We shall therefore have to assume 

 here, as with Vanessa prorsa, that it is not external influences 

 alone, which decide which primary constituent is to become 

 active, but that with a part of each brood from unknown internal 

 causes a tendency to the activity of, it may be, the summer 

 form, or, it may be, of the winter form, may be present from the 

 beginning. But this tendency to the summer form is always 

 connected with the tendency to immediate (subitaner)* develop- 

 ment, the tendency to the winter form with delayed (latenter) 

 development, i. e. if a pupa of the summer brood from wild speci- 

 mens does not develop immediately, but hybernates, then it 

 always produces the winter form, although it is kept in warmth 

 during the ivhole ivinter. On the other hand, all pupae of the 

 second brood, in case they emerge the same autumn, produce 

 instead the summer form ; Vanessa prorsa, at least, behaves in 

 this way. I have no experience with Pieris napi in reference to 

 this point, and Merrifield's experimentsf on it are not conclusive, 

 as he placed only ten pupae of the second brood immediately in 

 a high temperature (32° C), none of which happened to be 

 forced, but all hybernated. 



Now there might be an inclination at last to admit, that 

 temperature had no influence in the determination of the form 

 of the imago, and that all specimens of any brood, which assume 

 the winter form, do so from an innate disposition ; and the same 

 with those which assume the summer form. But such a view 

 would be refuted by the experiments. Bather the specimens of 

 the summer brood, which are ready for immediate emergence, 

 and at the same time for the activity of the sumnler form, can 

 have their intentions altered by cold, and be induced to assume 

 the winter form, although they do not hybernate, but emerge, as 

 soon as they are taken from the ice into a warm temperature. 

 They retain, therefore, their habit of immediate emergence, 

 although they have been altered to the winter form. This is 

 obvious, for example, from my Experiment 14 of 1872, in which 

 the iced first brood, when placed in a forcing-house after three 

 months, produced 60 butterflies of the winter form, while 34 did 

 not emerge, but hybernated and produced the winter form in the 

 next spring. Most of the 60 which developed immediately would 

 in all probability have emerged under normal conditions in June 



'''■ This expression, which I first used in reference to the Daphnidse, I 

 have made bold to use here also, as the process is the same as there, only that 

 there it is a question of the immediate (subitaner) or delayed (latent) de- 

 velopment of eggs, but here of pupae. Cf. Weismann, ' Beitriige zur Natur- 

 geschichte der Daphniden,' Leipzig, 1876-1879. 



f " Experiments on Temperature-Variation in Lepidoptera," in Proc. 

 Ent. Soc. Lond. 1894, Part I., April. 



T 2 



