13 THE KNTOMOLOGISI. 



Then, thirdly, we have in such species as A. villica, T. 

 proniiba, &c., examples of the final stage in the evolution of 

 3'ellow, — the stage when the pigment has become utterly in- 

 soluble (in so far, at least, as my reagents are concerned) and 

 very stable. 



Now let me show how this explains the phenomenon of the 

 metamorphosed red species. We may explain it from two stand- 

 points, — the chemical or the biological ; and at present I do not 

 perceive that we have warranty for pronouncing absolutely in 

 favour of either explanation. Let us take the biological first. 

 We may fairly assume that the evolution of a yellow pigment in 

 a formerly white species is — in its biological significance — a very 

 great advance. Now, without doubt, the further evolution of red 

 from yellow is another important advance, and (in this aspect) 

 would depend upon the " chance " variation of some individuals 

 to a red form, which red, being especially advantageous in some 

 way, would be preserved by Natural Selection. Now, the longer 

 that a species has been yellow the greater is the probability of 

 such fortuitous red variations occurring ;* conversely, the newer 

 that the yellow of a species is, the less opportunity has there 

 been given for a variation to red. In this light, then, we would 

 regard the existence of a normally red species as evidence that it 

 had passed through a long, long, period in the yellow stage. But, 

 according to preceding arguments, this is equivalent to saying 

 that the yellow has reached the thoroughly stable and non- 

 sensitive condition, which readily explains why a red colour 

 cannot be retrogressively modified further back than yellow. 

 From the simply chemical standpoint we should ignore the 

 action of Natural Selection as prime agent, and merely say that 

 the metabolic processes that originally led to the formation of 

 yellow pigment, and then to its further modification into a stable 

 yellow, tended to culminate in a red. This interpretation equally 

 supposes a stable yellow as the precursor of the red ; so that 

 from whichever standpoint we regard the subject, we must con- 

 sider the existence of red as a guarantee of the stability of its 

 predecessing yellow ; in either case we have an explanation of 

 tlie puzzle. 



But that the biological explanation (which, of course, equally 

 supposes the susceptibility of yellow to a red-producing change) 

 is safer than the purely chemical seems to me probable, not 

 only from various general considerations (including this, that it 

 is unwise to assume. what looks like an " inherent tendency" to 

 progress — at least without strong cause), but also from the fact 

 that the biological explanation can be better harmonised with the 

 exceptional beliaviour of Delias hierte and eucharis ; for in these 

 species the modification has been so rapid that the yellow has 



• In the concluding part of these articles I shall have occasion to quote a number 

 of instances illustrating this and allied points. 



