MIMETIC ASSOCIATIONS 



29 



yellow, and here we should have both forms occurring, the yellow or dominant form being 

 most numerous. Such a view of the appearance of the white suffusion would in some 

 measure reconcile the views of the authors quoted, albinus being regarded as an occasional 

 reversion to the white suffusion, which tendency the modification of chrysippus to dorippus 

 has not entirely eliminated. The comparative rarity of albinus seems to support this view. 

 Though we are as yet very imperfectly acquainted with the natural causes which determine 

 the transition of white to yellow or vice versa in the colours of Lepidoptera, such a variation 

 would seem to be fairly common. Other species, as for instance Amauris echeria, occur either 

 as yellow or white spotted forms without any very obvious reason for the variation. In 

 the paper on the chemical nature of lepidopterous pigments by Professor F. Gowland Hopkins, 

 already quoted, it is pointed out that in the group Pieridae an important point is the inter- 

 changeability between white, yellow, and orange, as shown by the colouring of closely 

 allied species, and even in variations of the same sex and species. These remarks apply, 

 it is true, to the Pieridae, to which group the forms now being considered do not belong, 

 and in which the pigments appear to be of a different constitution to those of other 

 families. They supply, however, another instance of the variation from yellow to white, 

 and, what is still more interesting to note. Professor Hopkins is of opinion that the white 

 colour is due to oxidation of the yellow taking place in the pupal condition, a suggestion 

 which reminds us again of Col. Yerbury's theory as to the origin of the white suffusion. 

 There are, however, many difficulties in the way of this latter proposition. Alcippus occurs 

 principally on the west coast of Africa, and unless it can be proved that the climatic con- 

 ditions there obtaining tend to produce oxidation more than those of other localities, there 

 is little to support the explanation. There is also another and greater difficulty. If the 

 white hind-wing is the product of the actinic effect of light on the pupa, why is not the 

 fore-wing affected, since in the pupal state it lies over the other, and should therefore be 

 the first to be acted upon ? 



It will be seen from the foregoing considerations that much still remains to be discovered 

 respecting the origin of these different forms. They are not due to the phenomenon known 

 as seasonal dimorphism, and there are difficulties in the theory of climatic conditions, 

 since in some localities the different forms occur together. We can, therefore, only conclude 

 that the species is of a somewhat unstable nature, liable to vary approximately on the lines 

 of the three principal varieties, and that natural selection has caused these three varieties 

 to become somewhat pronounced and fairly constant in some localities. Given that a species 

 is liable to variation, natural selection will guide such variation along the lines most cal- 

 culated to perpetuate the species. If the tendency to variation is slight, the original pattern 

 and colour may be altered in course of time, but there will not be a great liability to form 

 well-marked contemporary varieties. If, on the other hand, the species is of an unstable 

 nature and the liability to variation great, there will be greater and more frequent oppor- 

 tunities for natural selection to form well-marked varieties which can fill a new place in 

 the economy of nature. The very evident adaptability of D. chrysippus to varied circum- 

 stances is evidenced by its extraordinarily wide geographical distribution, and whilst all forms 

 may occur together in certain localities, the predominant form will be that best suited to 

 the immediate circumstances. Such circumstances are undoubtedly exceedingly complicated. 

 Whilst one insect may be modified so as to escape its enemies solely by its resem- 

 blance to a protected species, the latter in turn will be altered to suit other conditions 

 which in the light of our present knowledge are too obscure to be fully understood. 

 Apart, however, from the special advantages enjoyed by the different forms of D. chry- 



