XXV] DARWIN 3 



were brought on by the least mental excitement, which often 

 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, fortunately, 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 were 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 former similar occasion, his first 

 suggestion was, * You had better ask Wallace.' My difficulty 

 is, Why are caterpillars sometimes so beautifully and artistic- 

 ally coloured ? 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 any one 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 caterpillars, 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 West- 

 minster Review) my rather elaborate paper on " Mimicry and 

 Protective Colouring," and the numerous cases in which 

 specially 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 



