DARWIN 3 



rendered it impossible for him to see his friends, and which 

 appear to have lasted at intervals throughout his life. This 

 must always be remembered when we consider the enormous 

 amount of work he was able to do ; but, unfortunately, the quiet 

 interest of carrying- out observations or experiments lasting 

 for months, and often for years, seem to have been beneficial. 

 On the other hand, writing his books and correcting the MSS. 

 and the proofs in the very careful manner he always practised 

 was most wearying and distasteful to him. 



On February 23, 1867, he wrote to me asking if I could 

 solve a difficulty for him. He says : " On Monday evening I 

 called on Bates, and put a difficulty before him which he could 

 not answer, and, as on some similar occasion, his first sugges- 

 tion was, ' You had better ask Wallace.' My difficulty is, Why 

 are caterpillars sometimes so beautifully and artistically col- 

 oured? Seeing that many are coloured to escape dangers, I 

 can hardly attribute their bright colour in other cases to mere 

 physical conditions. Bates says the most gaudy caterpillar he 

 ever saw in Amazonia was conspicuous at the distance of 

 yards, from its black and red colours, whilst feeding on large, 

 green leaves. If anyone objected to male butterflies having 

 been made beautiful by sexual selection, and asked why they 

 should not have been made beautiful as well as their cater- 

 pillars, what would you answer? I could not answer, but 

 should maintain my ground. Will you think over this, and 

 some time, either by letter or when we meet, tell me what you 

 think?" 



On reading this letter, I almost at once saw what seemed 

 to be a very easy and probable explanation of the facts. I 

 had then just been preparing for publication (in the Westmin- 

 ster Review) my rather elaborate paper on " Mimicry and 

 Protective Colouring," and the numerous cases in which spe- 

 cially showy and slow-flying butterflies were known to have 

 a peculiar odour and taste which protected 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. 



