March 31, 1892] 



NATURE 



515 



although feeling that it is very difficult indeed to convey, 

 merely by a brief verbal definition, the practical distinc- 

 tions that one has slowly learnt from experience to recog- 

 nize. We will now take each of the colours in detail, 

 although, after this general account of the behaviour of 

 physical colours, there is not much left to say of blue, 

 or even of green. 



If now the tables of results be referred to, it will be seen 

 that I have arranged the blues in five different groups ; 

 but the differences between the first three — or probably 

 four — are so slight that they might almost as well be 

 thrown together. It is, however, somewhat convenient to 

 consider them apart. In the first group the blue is a 

 magnificent velvet blue, with a rich glow. Pritna facie, 

 it is evidently a physical colour (as Wallace, e.g., had 

 pointed out years ago), and its behaviour when tested 

 with reagents leaves no doubt of this. Reagents either 

 are without effect, or cause a temporary duUing which 

 disappears on drying, or plainly and permanently injure the 

 wing, and destroy the beautiful glow or even the colour 

 entirely. In cases of merely temporary dulling, where 

 the full colour returns on drying, I believe that the effect 

 is due simply to the soaking of the wing, and that neutral 

 liquids would produce the same effect. The second 

 group, after the explanations I have already given and 

 the information that I have tabulated, requires very little 

 comment. The various reactions abundantly showed that 

 all these are simply physical (interference) colours. The 

 third group are hardly distinguishable from the second : 

 the behaviour of the blue on P. macliaon when wetted with 

 a reagent and then dried, is an excellent example of such 

 physical colours as were referred to above. Now, con- 

 cerning the fourth group, which in all probability should be 

 considered as one with the three foregoing. I presume that 

 most people are familiar with our beautiful and common 

 Vanessa butterflies, the " Peacock," " Admiral," and 

 " Toi-toiseshell," and know that the borders of the wing 

 are marked in the two latter (as well as in the " Camberwell 

 Beauty ") with spots of blue, while in the " Peacock " there 

 are magnificent blue ocelli. The position of many of 

 these marks strongly reminded me of the special positions 

 of blue in various flowers ;^ and at the commencement of 

 my experiments I was in great.hopes of discovering a blue 

 pigment in these Vanessce ; but after repeated experiments 

 I was driven to conclude it almost certain that the blue 

 here is simply physical. Its reactions throughout indicate 

 as much ; since, on being treated with the reagents, it either 

 is wholly unaffected ; or it disappears, but returns on 

 drying ; or it pales to a sort of grey that resembles the 

 effect produced in the species of the third group ; or 

 lastly, it may in some cases disappear entirely, as I have 

 already pointed out that some physical colours may. 

 Finally, we have in the fifth group, containing the little 

 blue butterflies of the family Lyccefiidce, the only instance 

 I have found of a blue not certainly physical, and even 

 here the evidence is, I think, in favour of a physical 

 colour. The question, however, is an unusually per- 

 plexing one ; and for a long time I supposed that these 

 were pigment blues, but I am very doubtfal about them 

 now. There is no solution, and I have no evidence of 

 any reversion effect ; the colour is changed certainly, and 

 it is rather significant that in several of the deeper 

 coloured species the artificial colour thus obtained is 

 nearly identical with the normal colour of P. corydonj 

 but such changes in no way preclude the colour being 

 physical. The fact, too, that in several instances the effect 

 was to produce a green or greenish tint now appears to 

 me very suspiciously indicative of a physical colour (cf. 

 Papilio polyctor'xw. group 2). I may add, too, that the re- 

 action of the green in the closely related "Hairstreak" 

 butterfly, Thecla rubi, which I think is in all probability 

 physical, must also be taken into account ; for the reaction 



» Vide, or instance, Grant Allen's " Colours of Flowers." 



NO. I I 70, VOL. 45] 



in that instance is similar in general character to that of 

 these blues. 



To sum up, then, the case for this last group of blues, it 

 seems to me that we cannot certainly conclude them to 

 be physical, but the evidence points very stongly to the 

 view that they are— like the other blues — physical and 

 not pigmental. Should this conclusion be correct, I 

 have as yet found no instance of pigmental blue among 

 these Lepidoptera. 



We will now pass on to green. It will be seen that in 

 the tables I have divided green into three groups ; of 

 these, the first are unmistakable physical colours, exactly 

 analogous to the group of metallic blues, and it is there- 

 fore unnecessary to comment further on them. The 

 second group, though not metallic, are nevertheless, I 

 believe, also simple physical colours. Not only can I say 

 of them what was said of the blue LyccEnida — that 

 there is not the slightest evidence for any pigment ; but I 

 may go further, and say that there is some evidence for 

 the green being physical. The striking characteristic of 

 this group is that every reagent instantly turns the green 

 to a brown or bronze brown,' which reaction might, as 

 far as it goes, equally betoken either a pigment colour of 

 the " reversible " nature, or a mere physical colour. That 

 it is of the latter nature is indicated both by the fact that 

 I have observed, no true reversion effect (always defining 

 this reversion effect by the standard example of red), 

 and also since it is possible to produce a similar, though 

 only temporary, transformation by pure water or by 

 alcohol. This, I think, makes very strongly indeed for 

 the colour being simply physical, loth as I am to recog- 

 nize that the magnificent and interesting greens of such 

 species as the Argynnis fritillaries, and Thecla rtibi are 

 unpigmental. Still, my final conclusion, after prolonged 

 and careful consideration, is that these colours are simply 

 physical." 



Coming now to the third group of greens, we have 

 here undoubtedly pigment colours, showing the solution 

 effect. There are various degrees of solubihty among 

 them, and a varying sensitiveness to different reagents ; 

 but the summary, in brief, is that the green pigment is 

 dissolved out, leaving a white, i.e. unpigmented, wing. 

 Here, again, I need merely repeat what has already been 

 said of yellow, and will again be referred to, viz. that the 

 (green) pigment has been developed, not from a white 

 pigvietit, but in a white, z.^. unpigmented, wing. A further 

 question, however, arises — whether green has been directly 

 evolved as such, or is a second stage in the coloric evo- 

 lution. If the table be examined, it will be found that in 

 several cases the green has been transformed to yellow or 

 yellowish ; and this has occurred too commonly to be 

 otherwise than significant. I am therefore of opinion 

 that green has been evolved from yellow, and that the 

 production of yellow in these cases under the influence 

 of the reagents is a retrogressive metamorphosis compar- 

 able with the production of yellow from red. The evi- 

 dence admittedly is not anything like so conclusive or 

 copious for the inference of this derivation of green, and I 

 should, perhaps, hardly have advanced this view but for 

 the analogy to the standard behaviour of red. As it is, 

 however, it seems to me incumbent to hold — at least pro- 

 visionally — that these pigment greens have been evolved 

 from yellow.^ It is, however, very evident — as will 

 appear froin the following discussion — that the respective 

 relations of green and red to yellow are very different 

 indeed, although there be a community of descent. It 

 may be well to point out also that these greens occur in 

 three very different groups of the Macro- Lepidoptera, viz. 

 in the Rhopalocera, the Noctua;, and the Geometrae. The 

 apparent exception of Cidaria will be referred to later ; it 



' It is especially interesting that in T. rttbi this brown is the same as the 

 usual ^n»««rf colour, constituting the greater part of the wing surface. 



" A discussion in somewhat greater detail of this group— indeed, of the 

 greens in general— will be found in the Entomologist for May 1891. 



3 Cp. also Entomologist for May 1891. 



