THE INFLUENCES OF ENVIRONMENT 273 



(io°C.). Butterflies emerged which were not quite so black as those 

 of Naples, but considerably darker than the German form. Con- 

 versely, German pupae were exposed to greater warmth (38° C.), and 

 these gave rise to butterflies which were rather less flery gold and 

 considerably blacker than the ordinary German form. If I had to 

 repeat these experiments I should use a much lower temperature 

 in the case of the cold experiments, because we now know from the 

 experiments of Standfuss, E. Fischer, and Bachmetjeflf, that most 

 of the pupae of diurnal butterflies can stand a temperature below 

 zero for a considerable time ; probably the results would be even more 

 marked then. 



But even from the results of my former experiments we are 

 justified in concluding that the blackening of the upper surface 

 of the wing is really the direct result of the increased temperature 

 during pupahood, and that the pure red-gold results from the 

 lowered temperature. Similar experiments made by Merrifield with 

 English Phlcvas pupae agree exactly with mine. But we may con- 

 clude further from these experiments that both warmth and cold 

 only give rise to slight variations in the individual pupa^, and that 

 the pure red-gold of the northern form and the black of the southern 

 are the result of a long process of inheritance and accumulation, 

 in which the germ-plasm has been caused to vary in as far as the 

 relevant determinants are concerned, so that these yield the respective 

 northern and southern forms even in less extreme temperatures. 



As it is to be assumed that these determinants are present not 

 only in the primordium of the wing in the pupa, but also in the 

 germ-cells, both must be affected by the varying temperature, and, 

 in accordance with the continuity of the germ-plasm, each variation 

 of these determinants, however slight, would be continued in the 

 next generation. It is thus intelligible that somatic variations like 

 the blackening of the wings through warmth appear to be directly 

 inherited and accumulate in the course of generations ; in reality, how- 

 ever, it is not the somatic change itself which is transmitted, but 

 the corresponding variation evoked by the same external influence 

 in the relevant determinants of the germ-j^lasm within the germ-cells, 

 in other words, in the determinants of the following generation. 



This interpretation of these experiments, which I offered some 

 years ago, has been confirmed in several ways in regard to various 

 other diurnal Lepidoptera. By employing a temperature as low 

 as 8°C. in the case of fresh pup^e of various species of Vanessa 

 Standfuss and Merrifield, and especially E. Fischer, succeeded in 

 getting great deviations in the marking and colour of the full-grown 

 II. T 



