INTRODUCTION. v 



its former condition. Long exposure to these fumes, or the direct 

 action of liquid ammonia, renders the change comparatively permanent 

 and the yellow is, more or less, retained. A solution of sodium- 

 hydrate or potassium-hydrate will produce a similar result, and 

 probably all the fixed alkalies will give a permanent change. The white 

 of Parnassius and Lcticophasia, of (Eneis and of Syrichthus gives a 

 similar result. Here then is a decidedly pigmental white, capable of 

 being directly and rapidly influenced. Evidently, therefore, it differs 

 very widely in constitution from the white of Pieris. The white 

 colour in the fringes and undersides of certain Melitcece and Lyccence 

 (corydon, icarus, bettargus &c.), is as unstable and subject to exactly 

 the same rapid modifications as that of M. galatea &c., and is probably, 

 of exactly the same character. 



It now becomes easy to understand how readily " natural selec- 

 tion," environment &c., produce the bright orange varieties which the 

 undersides of Melanargia galatea so often exhibit in various localities, 

 and what an easy transition it is from pigmentary white to yellow. 

 At the same time, it throws a side light on the development of yellow 

 varieties of normally non-pigmentary species such as Pieris rapce var. 

 novanglice, Spilosoma menthastri var. scotica, the sexual dimorphic 

 coloration of S. lubricipeda and Hepiulus humuli, the yellow underside 

 varieties of Pieris napi var. jiava, the yellow male form of Fidonia 

 piniaria, which must be looked upon as the progressive form of the 

 white male, and endless other similar varieties. From the analogy of 

 M. galatea, I believe this change to be effected by the intermediary of 

 an unstable white pigment to which, however, I have not yet obtained 

 a clue. That such may exist is possible, when we consider the rapidity 

 with which some species have their yellow dissolved and how very 

 quickly they become transparent. 



But now for an actual intermediate white. A non-pigmental 

 white showing a tendency to produce yellow pigment is seen in many 

 female examples of EucJiloe cardamines on the upperside of the posterior 

 wings. The upper marginal area and the whole central area of these 

 wings are frequently strongly suffused and tinted with pale ochreous, 

 giving a distinct transition, whilst the development of the orange 

 (sometimes yellow) tip to the male, is a further stage in the same 

 direction, the apparent green on the underwings is in reality not green 

 but yellow and black, and hence we see in every direction the tendency 

 in this species to a yellow coloration. I may add that the orange- 

 yellow is excessively soluble wherever developed in this species, and 

 suggestive of comparatively recent (if not actually present) develop- 

 ment. It is interesting to note, too, that occasionally the male has 

 only the " orange tip " to one wing, and that its size &c., vary much. 

 The allied E. eupheno shows distinctly this natural advance, and ve 

 have a species, where the suspicion of ochreous in cardamines has be- 

 come permanent and the orange has a tendency to become red, thus 

 showing the natural genetic relationship of these colours. It now 

 becomes clear that there is a direct tendency for white to become 

 modified into yellow, and we find many white species have forms, 

 graduated from the purest white through faint ochreous to a brilliant 

 yellow, as in the species I have just mentioned, and that this rapidly 

 takes place where " natural selection " or environment demands it. 

 This influence of " natural selection " is very prominent in the upper- 



