( Ixxix ) 



I will, iu the first place, appeal, as I did last year in 

 connection with the general subject of speculation in biology, 

 to the green colour of leaf-feeding larvae. It cannot be sup- 

 posed that this colour is of direct physiological use in the 

 same way that it is of use in the leaf. If it has any use at all, 

 and I think that this has been shown to be the case, it is for 

 the purpose of concealment. Among the first entomological 

 guesses I ever attempted in my younger days was the sug- 

 gestion that the colouring-matter of the leaf had in such cases 

 been utilised. That guess was converted into a demonstrable 

 fact by the experiments of my friend, Prof. Poulton, which 

 are now so well known as to require no recapitulation. But 

 the verification of my crude guess carries with it very wide- 

 reaching consequences, which bear directly upon the present 

 subject. If this character is of use, and is the result of the 

 action of natural selection, it must have arisen from physio- 

 logical variability, i.e., out of generations of leaf- feeding 

 larvfe belonging to various orders of insects there have sur- 

 vived those groups whose digestive arrangements were capable 

 of allowing chlorophyll to pass in a modified form into the 

 blood, and so to colour the larva. A physiological process of 

 a most remarkable kind has here been called into existence 

 because it is correlated with a useful external character. 

 What particular organ or organs have had their functions 

 modified for this purpose is a question of the greatest physio- 

 logical interest, but does not immediately concern us now. The 

 fact that the colour is possessed by larvae belonging to different 

 orders of insects points to a community of character to start 

 from, and this is just what might have been anticipated with 

 respect to such fundamental functions as those of a physio- 

 logical nature. On the other hand, it may be necessary to 

 point out that chlorophyll does not furnish the only road to 

 green coloration in animals, because there are many green 

 species in which this substance cannot have been utilised in 

 the way that has been indicated. 



From community of internal function, such as may fairly 

 be regarded as the characteristic of physiological processes, 

 it is not a very rash jump to the suggestion that all or many 

 of those causes oi species transformation which have been 



