82 ANIMAL COLORATION. 



(p. 77) with reference to tlie Arctic foxes at the Zoological 

 Gardens. Professor Weismann goes on to show reasons why 

 umbrosa and not Fabricii was really the winter form (they 

 neither of them pass the winter in the pupa form) ; hence 

 the effects of cold upon the pupse are more intelligible. 



It is not only butterflies which exhibit a seasonal dimor- 

 phism ; the same phenomenon is exhibited by certain moths. 

 One of the " thorn " moths (Selenia illustraria) is an example ; 

 Mr. Merrifield has published in a recent part of the Transac- 

 tions of the Entomological Society a good account, accompanied 

 by a plate, of his experiments with this insect and with 

 Ennomos autumnaria-tiliaria. 



By icing (a temperature of 33°) pupte of illustraria of the 

 summer brood, a darker colour was produced upon the resulting 

 imagos ; on the other hand, by forcing (at a temperature of 80°) 

 pupas of the summer brood, moths emerged which were inter- 

 mediate in colour between the summer and spring broods, i.e., 

 paler than they would normally have been. 



There are various interesting questions concerning seasonal 

 dimorphism, the discussion of which would be beyond the 

 limits of this book : I am only desirous of pointing out here 

 that colour change may be directly caused by environmental 

 causes ; that this is so with seasonal dimorphic Lepidoptera 

 there can be no doubt. 



