258 



Influence of environment on animals 



determine natural death are certainly not identical with the pro- 

 cesses which underlie their development. T. B. Robertson has also 

 arrived at the conclusion, for quite different reasons, that the process 

 of senile decay is essentially different from that of growth and 

 development. 



(b) Changes in the colour of butterflies produced through the 

 influence of temperature. 



The experiments of Dorfmeister, Weismann, Merrifield, Standfuss, 

 and Fischer, on seasonal dimorphism and the aberration of colour in 

 butterflies have so often been discussed in biological literature that 

 a short reference to them will suffice. By seasonal dimorphism is 

 meant the fact that species may appear at different seasons of the 

 year in a somewhat different form or colour. Vanessa prorsa is the 

 summer form, Vanessa levana the winter form of the same species. 

 By keeping the pupae of Vanessa prorsa several weeks at a tempera- 

 ture of from to 1 Weismann succeeded in obtaining from the 

 summer chrysalids specimens which resembled the winter variety, 

 Vanessa levana. 



If we wish to get a clear understanding of the causes of variation 

 in the colour and pattern of butterflies, we must direct our attention 

 to the experiments of Fischer, who worked with more extreme 

 temperatures than his predecessors, and found that almost identical 

 aberrations of colour could be produced by both extremely high and 

 extremely low temperatures. This can be clearly seen from the 

 following tabulated results of his observations. At the head of each 

 column the reader finds the temperature to which Fischer submitted 

 the pupae, and in the vertical column below are found the varieties 

 that were produced. In the vertical column A are given the normal 

 forms : 



