404 THE GERM-PLASM 



If the area of distribution of the species extended unin- 

 terruptedly from the Polar regions to South Italy or North 

 Africa, all the intermediate forms would occur, from the pure 

 reddish-gold single-brooded form in Lapland to the black double- 

 brooded variety eleus : there would thus be — first, two similar 

 reddish-gold broods ; then similar ones, — those butterflies which 

 are exposed to a higher temperature during the pupal stage 

 having a tendency to develop a black tint ; and then seasonal 

 dimorphic forms, the butterflies being black in the summer and 

 reddish-gold in spring, as is actually the case in Genoa. A 

 still longer action of a higher temperature would at first change 

 a small and then an increasing number of determinants for the 

 scales into the ' black' variety, so that finally two broods, both 

 consisting of the black form elens, would occur. The case of the 

 Neapolitan colony is somewhat similar to the last-mentioned 

 one, for many black specimens certainly occur amongst the 

 spring brood, although there are also many light-coloured ones ; 

 none, however, are as light as the northern reddish-gold form of 

 P. phlceas. I do not know whether a complete change of colour 

 has been attained by both annual broods in any locality ; but if 

 it has, I should expect it to be seen in Southern Japan rather 

 than elsewhere, for the butterflies which I possess from the 

 neighbourhood of Tokio display an unusually dark colour. 



This case has been discussed at length because it appears to 

 me to be especially significant, not only in the explanation of the 

 climatic varieties of butterflies, but also as regards the theory of 

 heredity, and the assumption of material determinants which 

 exist in the gerin-piasm and are passed on from one genera- 

 tion to another. The facts are so evidently in favour of this 

 assumption that no other explanation seems possible. It must, 

 however, be remembered that the artificial modification of the 

 colour on the wings does not take place if the change in tem- 

 perature occurs only when the scales begin to become coloured. 

 The colouring matter is consequently not produced by the direct 

 influence of chemical transformations, but by an indirect influ- 

 ence, which we may suppose to be due to a mutual disarrange- 

 ment and rearrangement of the ' biophor-material ' of which the 

 determinants consist, by the co-operation of which latter the 

 chemical process forming the colour is derived. 



The occurrence of seasonal dimorphism alone, shows with 

 certainty that the determinants for the scales are influenced by 



