WARNING- COLOURS 167 



once that they were a difficulty in the way of the 

 theory. For caterpillars are undeveloped organisms ; 

 they have been described as ' embryos leading an in- 

 dependent life,' and there is no way of distinguishing 

 the sexes by external colour or structure (except in a 

 few instances). Here, therefore, we meet with bril- 

 liant colours, often rendering the possessors con- 

 spicuous, which cannot be of any use in courtship. 

 Seeing, therefore, that the bright colours must be of 

 use in some other way, Darwin drew the attention of 

 Wallace to the subject, and asked whether he could 

 suggest any explanation. Wallace accordingly thought 

 over the subject, and considered it as part of the 

 wider question of the varied uses (other than sexual) 

 of brilliant and startling colour, in other stages of 

 insect-life, and in numerous instances scattered over 

 the whole animal kingdom ; and he finally ventured 

 to predict that birds and other enemies would be found 

 to refuse such conspicuous caterpillars if offered to 

 them. He believed, in fact, that such larvae are pro- 

 tected by possessing a nauseous taste or smell, or some 

 other property which renders them unfit for food. 

 Conversely Wallace argued that inconspicuous cater- 

 pillars would be eaten and relished whenever they 

 were detected. 



It is most inspiring to read the letter in which the 

 great founder of modern biology accepted this fruitful 

 suggestion. 



' . . . You are the man to apply to in a difficulty. 



