274 THE EVOLUTION TIUOOHY 



insects, — so-called iiborrations, such us had previously lioen fouud only 

 \cry rarely and niui^iy under natural conditions. The <leviationa 

 from the normal nnist undouhtedly he ascribed to the t^H'ect ot colil, but 

 it does not follow that tliey are uvw forms \\'hich have suddenly sprunj;' 

 into existence, as uuvny ]ia\e hssuuuhI without turther i^\])eriment, 

 Dixey, on the other hand, has atten\pted to establish, by a comparison 

 of the difi'crent species of )'((/(r,svs((,, the phyletie di'xclopment of their 

 markings, and has found that these aberrations due to cold ar(^ more 

 or less comi)iete rt'X'ersions to earliei- phyletie staf;'es. As re{;iirds (lie 

 common suuill painted lad}' {VdiicKHii cardui), the small tortoise-shell 

 butterfly {Vuncfffu uiiicn'), the 'Admiral' (Va iicns((, (italaiili(), the 

 peacock (ViCiicfun lo), and the large tortoisi^-shell {]'(tiirss(i. 

 jMlychidroti), 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 generations of seasoiuUly dimo)'phie Lc^pi- 

 doptera might lie considered as a reversion. But tiiis by no means 

 excludes the possibility that other than atavistic aberraticms may 

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

 retically. ^'et we must not, without dm^ consideration, compare thes(^ 

 abruptly occurring variations to the sport-varieties of plants which 

 we have already discussed ; there is an important dilierence bt^tween 

 the two sets of cases. In the Lepidoptera a single interi'erenee, 

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

 plant varieties the visible appearance of the variation is preceded 

 l)y a long period of pre])aratory change within the germ-plaam. 

 This period required for the extm-nal inlhu'nees to take etfect was 

 already recognized by Darwin, and it has rc^cently \>vv\\ named by 

 De Vries the ' i)rennitation period.' 



We nuxy explain these remarkable aberrations tlie()r(^tically in 

 the following way: The determinants of the wing-scales in the wing- 

 primordium of the young pupa are inlluenced by the cold in ditlei'ent 

 ways, some kinds of di^terminants beting strtuigthened by it, others 

 mai-kedly weakened, even crippled so to speak, and in this way one 

 c(jlour-area si)reads itself out more than is nonnul on the sin'fae(? 

 of the wing, and another less, while a third is suppi-essed altogetiier. 

 That this disturbance of the (equilibrium betwcc^ii the detei'minants 

 leads usually to the development of a pliyletically older marking 

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

 modern species of Wonennu a certain nuud)er of (h^terminants of 

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

 We might even in(|uire whether these weie not bettc^r able to ondui'e 

 cold than their uioderu d(!Hcendants, sinc(e their oi-iginal possossoi's, 



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