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. 
(6) 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 0° 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 : 
A. to) Oo go 
0° to - 20°C] O° to+ 10°C. | (Normal eA ee ae ee 
ichnusoides | polaris urticae tchnusa polaris - tchnusoides 
(nigrita) (nigrita) 
antigone Sischert to — fischert antigone 
(iokaste) (okaste) 
testudo dineyt polychloros | erythromelas | dixeyi testudo 
hygiaea artemis antiopa epione artemis hygiaea 
elymi wiskotte cardut —_ wiskotti elymi 
klymene merrifieldi | atalanta — merrifieldi | klymene 
weismannt | porima prorsa — porima weismannt 
