176 
COLOUR IN NATURE 
CHAP. 
question of artificially produced colour-change is too 
vast to be entered into in detail here, this chapter 
would be incomplete without some reference to it. 
As the observations of Mr. Poulton and others have 
rendered the adaptive changes of caterpillars tolerably 
familiar in this country we shall not discuss them, 
but rather note some points in connection with the 
changes produced in the butterflies of the genus 
Vanessa by subjecting the pupae to varying condi¬ 
tions of temperature ; these being not quite so well 
known. 
The occurrence of seasonal dimorphism in the 
butterfly formerly described under the two specific 
names of Vanessa prorsa and V levana is well known, 
and the fact that similar colour-variation may be 
produced in other Vanessae by subjecting the pupa 
to varying temperatures is also familiar through the 
researches of Weismann, Fischer, and others. The 
interpretation of the results obtained has always, 
however, been a matter of considerable difficulty. 
Weismann’s conclusion in the case of Vanessa levana- 
prorsa is, or was, that each pupa contained within it 
the potentialities of both forms, and that the tempera¬ 
ture was not the efficient cause of the one produced, 
but merely the stimulus which set in motion the 
necessary pre-existing mechanism. Fischer made 
an extended series of observations on the same 
subject, and in the main agrees with Weismann, 
while Urech (1896) dissents very strongly from this 
position. We cannot here enter into detail on the 
subject of Urech’s paper—it is largely theoretical in 
nature—but may mention a few of his points. His 
main object is to prove that the differences between 
