402 THE GERM-PLASM 



We know, however, of a fact which definitely proves that the 

 susceptibility of the scale-determinants to the influence of 

 temperature is greatest at a certain stage in the development 

 of the butterfly — much greater than either before or afterwards. 

 I have frequently noticed in seasonally dimorphic species like 

 Vanessa prorsa-levatia^ that the modifying influence of heat 

 or cold only acts at the begmni?ig of the pupal stage. 

 Although I have not yet been able to ascertain the time at 

 which this occurs more precisely, it can be definitely stated that 

 the winter pupae of Vanessa levatia, for instance, which have 

 been exposed to a high temperature even only a month after 

 entering the pupal stage, are never transformed into the prorsa 

 form ; they all emerge as V. lev ana. 



This is not due to the fact that the colour of the wings is 

 already deposited a month after the insect has entered the pupal 

 stage, for at this time there is no trace of colour whatever. 

 There 7nust, consequently, be a period in the disintegration of 

 the determinants when they are most susceptible to the iti/lueftces 

 of temperature : subsequently this is no longer the case, and 

 although they are susceptible before this period, I nevertheless 

 venture to suppose that they were so to a far slighter extent. 

 This may be due to their connection with other determinants, 

 or to other causes which we are not yet able to discover. 



If, then, the determinants for the scales are only influenced 

 very slightly by the tem.perature as long as they are situated in 

 the germ-plasm, and are subsequently greatly affected by it at 

 a certain period in the development of the wings, the above- 

 mentioned phenomena admit of a simple explanation. The 

 germ-plasm of the southern colony of P. phlceas must contain 

 many determinants among those for the wings, which, in con- 

 sequence of the exposure of thousands of generations to heat, 

 have been adapted for the production of black scales, together 

 with a large number of others which only require a small 

 increase of temperature during pupation in order to give rise 

 to a black colour. These latter kind cause such fluctuations 

 in the coloration as occurred in my experiments ; while the 

 former produce the black coloration of the wings, which has 

 become fixed in the constitution of the southern colony, and 

 can no longer be removed by the action of cold on the young 

 chrysalis. 



In this case it is taken for granted that the ancestral form 



