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 a.nd 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 



