The Wallace-Darwin Correspondence 



* Mimicry and Protective Colouring/ and the numerous 

 cases in which specially sliowy and slow-flying butterflies 

 were known to have a peculiar odour and taste which pro- 

 tected them from the attacks of insect-eating birds and 

 other animals led me at once to suppose that the gaudily 

 coloured caterpillars must have a similar protection. I 

 had just ascertained from Mr. Jenner Weir that one of 

 our common white moths (Spilosoma mcnthastri) would 

 not be eaten by most of the small birds in his aviary, nor 

 by young turkeys. Now, as a tvhite moth is as conspicu- 

 ous in the dusk as a coloured caterpillar in the daylight, 

 this case seemed to me so much on a par with the other 

 that I felt almost sure my explanation would turn out 

 correct. I at once wrote to Mr. Darwin to this effect.'' 



Down, Bromley, Kent, S.E. February 26, 1867. 



My dear Wallace, — Bates was quite right, you are the 

 man to apply to in a difficulty. I never heard anything 

 more ingenious than your suggestion, and I hope you may 

 be able to prove it true. That is a splendid fact about the 

 white moths ; it warms one's very blood to see a theory 

 thus almost proved to be true.^ With respect to the beauty 

 of male butterflies, I must as yet think that it is due to 

 sexual selection ; there is some evidence that dragonflies 

 are attracted by bright colours; but what leads me to the 

 above belief is so many male Orthoptera and Cicadas 

 having musical intruments. This being the case, the 

 analogy of birds makes me believe in sexual selection with 

 respect to colour in insects. I wish I had strength and 

 time to make some of the experiments suggested by you ; 

 but I thought butterflies would not pair in confinement; I 

 am sure I have heard of some such difficulty. Many years 

 ago I had a dragonfly painted with gorgeous colours, but 

 I never had an opportunity of fairly trying it. 



^ I.e., the suggestion that conspicuous caterpillars or perfect insects (e.g. 

 white butterflies) which are distasteful to birds are protected by being easily 

 recognised and avoided. 



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