﻿CHEMISTRY OF INSECT COLOURS. 157 



Now regarding this progressive modification of colour, — 

 although carefully guarding myself against expecting too much, 

 or admitting so sweeping an anticipation as that of producing 

 artificially all the stages in coloric development displayed in 

 nature, seeing that such are probably due to the action of more 

 causes than simply the metabolism of the insect ; due, for 

 instance, to variations of temperature, atmospheric and climatic 

 conditions generally, — yet I considered that by the external 

 application of such already mentioned organic reagents, as in 

 very small and dilute quantities are present in the insects them- 

 selves, I might produce progressive modifications of colour 

 similar to those that have occurred already in many species. I 

 hoped thus to complete changes already commenced in an insect 

 itself, or displayed by closely allied species : for instance, to 

 change the ivhite of Pieris into the black, already present as 

 spots and tip markings ; the white of Euchloe cardamines into 

 orange, like the tip ; the pale yellow of Colias hyale into the 

 deep yellow of Edusa ; these yellows into black, like the borders ; 

 and the yellow of Gonepteryx rhamni into the red of G. cleopatra. 

 Especially, too, in those species where the males are brilliantly 

 coloured and the females dull, did I anticipate being able to 

 change the female tint into the male (cf., Lycsense, Euchloe, &c.).* 

 How utterly fallacious and doomed to disappointment were all 

 these anticipations, will subsequently appear. 



Then as to the retrogressive modifications, — here, at any rate, 

 the event proves me to have been on somewhat safer ground in 

 my predictions, I have already explained that I expected to find 

 such reagents as the strong mineral acids and the caustic alkalis 

 convert the latest evolved colours backwards to a lower colour, 

 thus giving me the original (sic) type colours ; and this 

 both as regards the several colours of one species, and the 

 range of colours in a genus of many species. These results 

 would form, therefore, an exact counterpart to those that I 

 fallaciously hoped for in progressive modification. For instance, 

 in Pieris I expected to turn all the black into white; in Euchloe 

 cardamines, the orange back to white; in Gonepteryx rhamni, the 

 red spots to yellow; and in Vanessa atalanta, the red into the 

 ground colour, black. 



Supposing these anticipations to be fulfilled, the question 

 next occurred to me whether it might be possible by diluting a 

 given reagent, or by taking a less strong one, to retrace retro- 

 gressively the colour evolution step by step. For example, — I 

 thought, — a powerful acid may turn Colias all white at once ; 

 but it would be far more interesting if (by using weaker acid) one 



* In tin's way, too, I hoped to throw some light on various isolated observations 

 regarding the effect of change of diet in producing coloric variation ; as, e.y., that 

 Arr tin caia is far darker when the larvie are led on walnut leaves. 



