XXI 



higher or lower temperature. The same fact occurs also sometimes during 

 particular conditions of the temperature in nature. Thus in these proofs it has 

 always been a matter of abnormal phenomena, which for this reason showed 

 an absolutely morbid character when they were brought about artificially in 

 a higher degree than ever happens in nature, and which lead to the remarkable 

 result that both, by strong heat or severe cold, the same changes are caused. 

 A clear proof, indeed, that it is not any longer the specific character of the 

 artificial influence — in this case cold or heat — which causes the effect, but only 

 the fact of the irritation exercises this influence, and that as soon as this has 

 passed certain limits, it simply causes a morbid disturbance. Though thus 

 the said climatical influences may, to a certain degree, affect the progress of 

 an existing process of evolutional change, the alterations which are brought 

 about by that progress are, not directly, however, caused by those influences. 

 The later researches by Pictet showed, moreover, that even such an influence, 

 which can really cause some alteration of colour, particularly certain food of 

 the caterpillars, cannot take an active part in it for more than a few generations ; 

 after that time it seems that the insects have so much adapted themselves to 

 the new food that this can no longer affect their colour, and that they again 

 take their origanal tint. Here also the external, or rather the direct influence 

 only causes an abnormal, if not a morbid condition, which can only exist tem- 

 porarily and so can never be the cause of normal and therefore lasting diffe- 

 rences of colour. 



Among caterpillars also the phenomenon of colour-evolution occurs. I 

 observed it first among the larvae of Sphingidae and since then I have wit- 

 nessed the same dimorphism in which this phenomenon reveals itself in those 

 caterpillars, also among other larvae, between which there was a great difference, 

 as well in caterpillars of Java, Satyridae f. i., as with those of Eupithecia. 

 Evidently the evolutional change only takes place in respect to the ground colour 

 and with regard to the increase of the black, which occurs among the Pieridae 

 in the caterpillars of Callidryas Pomona F. So it is obvious that just as 

 the butterflies have pigmental as well as structural colours, of which only the 

 former are subject to the colour-evolution, those same two kinds of colour also 

 exist in the caterpillars, and that the ground colour must be considered the 

 pigmental one. But as regards this, it seems that only very little has been 

 investigated as yet, and, therefore, there is not much to be said about the 

 said phenomenon in caterpillars. Where, however, a case of this character 

 occurs, I shall draw attention to it. 



As to the chrysalids, I think we must also admit the existence of colour- 

 evolution. Very important researches, especially those by Poulton, have proved, 



