288 Studies in the Theory of Descent. 



biological value, they must be attributed to the 

 action of natural selection. 



Did I here at once proceed to establish the 

 biological value of larval markings- and especially 

 of those of the Sphingidce so as to arrive in this 

 manner at a conclusion as to their dependence upon 

 natural selection, it would be impossible to avoid the 

 consideration of the total coloration of the cater- 

 pillars, since the marking frequently consists only 

 of a local strengthening of the colour, and cannot 

 be comprehended without coming to this under- 

 standing. The action of the markings also often 

 appears to be opposed to that of the colouring, 

 making the caterpillar again conspicuous ; so that 

 the two factors must necessarily be considered to- 

 gether. I shall therefore commence the investi- 

 gation with colour in general, and then proceed to 

 treat of marking. 



