Appendix. 533 



at various stages of growth, and observing whether any 

 change of colour was produced. In accordance with the 

 principles advocated in a previous note (p. 305), it is 

 conceivable a priori that phytophagic variability may 

 occur by direct chemico-physiological action, quite 

 irrespective of any of the changes of colour being of 

 protective use. In the case of brightly-coloured dis- 

 tasteful species phytophagic variability might thus have 

 full play, but in the case of protectively-coloured edible 

 species, phytophagic variability would be under the 

 control of natural selection. These considerations raise 

 a question of the greatest theoretical interest in con- 

 nection with this phenomenon. If phytophagic varia- 

 bility can have full play uncontrolled by natural selec- 

 tion in brightly- coloured caterpillars, ought not this 

 phenomenon to be of more common occurrence in such 

 species than in those protectively coloured ? Although 

 our knowledge of this subject is still very imperfect, as 

 a matter of fact brightly coloured larvae, so far as I have 

 been able to ascertain, do not appear to be susceptible 

 of phytophagic influences. But this apparent contra- 

 diction, instead of opposing actually confirms the fore- 

 going views, as will appear on further consideration. 

 The colours of protected species are as a whole much 

 inferior in brilliancy to those of inedible species, so that 

 any phytophagic effect would be more perceptible in the 

 former than in the latter, in which the highest possible 

 standard of brilliancy appears in most cases to have been 

 attained. Now phytophagic variations of colour appear 

 to be of but small amount, or, in other words, such 

 variations fluctuate within comparatively restricted limits, 

 and as the cases at present known are mostly adaptive 

 it is legitimate to conclude that they have been pro- 

 duced and brought to their present standard by natural 

 selection, /. e. that they have arisen from phytophagic 

 influences as a cause of variability. The initial stages 



