274 THE EVOLUTION THEORY 



insects, — so-called aberrations, such as had previousl}' been found only 

 very rarely and singly under natural conditions. The deviations 

 from the normal must undoubtedly be ascribed to the effect of cold, but 

 it does not follow that they are new forms which have suddenly sprung 

 into existence, as many have assumed without further experiment. 

 Dixey, on the other hand, has attempted to establish, by a comparison 

 of the different species of Vanessa, the phyletic development of their 

 markings, and has found that these aberrations due to cold are more 

 or less complete reversions to earlier phyletic stages. As regards the 

 common small painted lady (Vanessa, cardud), the small tortoise-shell 

 butterfly (Vanessa urticoi), the 'Admiral' [Vanessa atalanta), the 

 peacock (Vanessa io), and the large tortoise-shell (Vaaes)iu 

 'polycliioros), I can agree with this interpretation, and I do so the 

 more readily because some years ago I suggested that the alternation 

 of differently coloured genei-ations of seasoiially dimorphic Lepi- 

 doptera might be considered as a reversion. But this by no means 

 excludes the possibility that other than atavistic aberrations may 

 be produced by cold or heat. There is nothing against this theo- 

 retically. Yet we must not, without due consideration, compare these 

 abruptly occurring variations to the sport-varieties of plants which 

 we have already discussed ; there is an important difference between 

 the two sets of cases. In the Lepidoptera a single interference, 

 lasting only for a short time, modifies the wing-marking, but in the 

 plant varieties the visible appearance of the variation is preceded 

 by a long period of preparatory change within the germ-plasm. 

 This period required for the external influences to take effect was 

 already recognized by Darwin, and it has recently' been named by 

 De Vries the ' premutation period.' 



We may explain these remarkable aberrations theoretically in 

 the following wa}" : The determinants of the wing-scales in the wing- 

 primordium of the young pupa are influenced by the cold in different 

 ways, some kinds of determinants being strengthened by it, others 

 markedly weakened, even crippled so to speak, and in this way one 

 colour-area spreads itself out more than is normal on the surface 

 of the wing, and another less, while a third is suppressed altogether. 

 That this disturbance of the equilibrium between the determinants 

 leads usually to the development of a phyletically older marking 

 pattern leads us to the conclusion that in the germ-plasm of the 

 modern species of Vanessa a certain number of determinants of 

 the ancestors must be contained in addition to the modern ones. 

 We might even inquire whether these were not better able to endure 

 cold than their modern descendants, since their original possessors, 



