COLOUR IN LEPIDOPTERA, 7 



and yellow ou butterflies does no more bear an accidental 

 character than that of the colours of Sphingidae-caterpil- 

 lars, but the change is apparently always going on under 

 the action of influences representing a general, fixed prin- 

 ciple. So the brilliant optical colours occasioned by a mor- 

 phological change of the scales also appear where the 

 development of dark pigment is small, while these scales 

 contain a great deal, if not solely, either red or yellow 

 pigment ; so making it rather difficult to admit that those 

 pigmental colours can be going side by side with the 

 structural colours, the latter making their appearance in 

 order to fill up the place of the former. Nor can this 

 theory solve the many biological questions, presenting 

 themselves on account of the colour of butterflies, which 

 my theory does in a full measure. 



By comparison of series, first of individuals of the same 

 species, then of those very little differentiated species which 

 are probably only races of the same species, and further 

 of species belonging to the same genus, it became quite 

 clear to me that, with imagines of Pieridae at least, there 

 is a continual change of colour going on in a definite di- 

 rection, exactly concurring as a phenomenon with that 

 observed in the caterpillars of the Sphingidae, and in no 

 wise showing an accidental character or one connected 

 with the surrounding. And further that this change consists 

 in a course from red through orange and yellow into white 

 or in reversed order, during which course often — but 

 not always — black also appears more or less, but dis- 

 appears again afterwards. And by a continued study of 

 this course and of what reveals itself as being from an 

 earlier or a later origin, I have got the conviction that it 

 is in the first indicated order — the one in which red 

 appears as the primary colour — that this evolution goes 

 on in the above named family. 



Once having asserted this, I have been able to inter- 

 pret a great many phenomena which hitherto had not 

 been explained in a satisfactory manner. And since this 



Notes from tlie Leyden Museum, Vol. XXII, 



