=^16 



NATURE 



[March 31, 1892 



is just possible that in these species the green is 

 descended from, not yellow, but chestnut. 



Quitting the greens,^ we now come to what is out and 

 away the most satisfactory and interesting colour that I 

 have studied — that is to say, red. Owing, however, to 

 the very intimate relations of this colour to yellow, it is 

 difficult to discuss them apart, and we will therefore take 

 yellow and red together. Referring, now, first of all to 

 the table of reds, what do we find as the general result ? 

 Omitting for the present (since they must be considered 

 later) the last four species, we find that in practically 

 every instance red is (rapidly or imtatttly) changed by 

 acids - to some kind of yellow or orange ; or, to state it in 

 terms of the views that I have been led to adopt, red is 

 retrogressively modified into the yellow from which it 

 was originally evolved.^ Here, however, the change 

 stops; for, with the one striking and interesting excep- 

 tion of Delias (and perhaps one might add one or two of 

 the pale pinks occurring among the Sphinges), the yellow 

 thus produced is immovable. And since the species 

 experimented upon include all varieties of red, and repre- 

 sent all the groups of Macro-Lepidoptera, one might 

 apparently conclude that, although red is an exceedingly 

 sensitive colour, yellow can never be affected. Yet, if 

 the table of the normally yellow species be examined, it 

 will be found that, in an immense number of these, the 

 yellow is either partially or wholly dissolved by various 

 reagents, leaving a pure white wing. Here, therefore, 

 we find ourselves at once face to face with the problem 

 of the character of the yellow pigment, and to a con- 

 sideration of that we must turn before proceeding further 

 with red. 



It will be observed that in this table of yellows I have 

 divided the species examined into four groups. Omitting 

 for the present the small second group, we may dis- 

 tinguish three stages, represented by the three groups i, 

 3, and 4.* This division has been adopted in order to 

 illustrate what seems to me the most feasible explana- 

 tion—at least for the present— of the constitution and 

 behaviour of this yellow pigment. In the first group the 

 yellow is exceedingly soluble, and a colourless white wing 

 is the result. In the next stage (the third group) the 

 yellow is more or less affected— sometimes very little 

 moved, sometimes finely dissolved. In the last group 

 the yellow is wholly insoluble attd entirely unaltered. 

 Also be it noted that in group i pale light yellows pre- 

 dominate, while in the last gi'oup the yellow is chiefly 

 orange. It is, further, clear from this that a complete 

 classification of all the yellows would include in this 

 fourth group all the yellows artificially produced by re- 

 acting on the reds. Now, the explanation which I have 

 adopted in order to cover all these facts is as follows. 

 It appears that the yellow pigment, when first ^ evolved, 

 is exceedingly sensitive and susceptible of evolution by 

 various reagents ; in this stage, too, it is probably of a 

 comparatively pale or light yellow colour. In course of 

 time the yellow pigment may in various instances become 

 slightly altered in constitution (generally accompanied by 

 a change to a deeper or more orange tint), and altered in 

 the direction o{ greater stability ; or rather, to confine our- 

 selves to the literal facts, altered to the extent of becom- 

 ing far less soluble. Of this intermediate stage we have 

 examples in group 3. Finally, in group 4 we have 

 examples of the last stage of evolution, when an— usually 



1 I have not thought it worth while to refer here to the pseudo-green of 

 Euchloa cardamimis : vide Entomologist, May 1891. 



^ It is unnecessary here to regard the less marked and less interesting 

 alkaline reaction. 



3 Some remarks on this subject will be found in the Entomologist, xxiii. 

 370-71- 



^ It is, of course, to be understood that, like all rigid divisions, this is to 



some extent artificial. Evidently Nature knows n )thing of three or four 

 sharply- circumscribed groups of yellow, but merely an ind finite series, of 

 which the first members would fall into my first group, and the last into my 

 fourth, and so on. 



5 I speak, of course, in a phylogenetic sense. 



NO. I 170. VOL. 45] 



deep-coloured — insoluble yellow has been evolved. It 

 appears to me, therefore, that usually red is evolved only 

 after a long apprenticeship of yellow, and this is as much 

 as to say that as a rule the yellow has become stable and 

 insoluble before its evolution into red : this explains why 

 red can be converted into yellow, but usually no further. 

 On the other hand, the striking instances of Delias and 

 one or two pink species show that occasionally the deve- 

 lopment of red has been so rapid that the yellow had not 

 previously become stable.^ The very parallel examples 

 of Cardaminus edtisa and Lyccena phlceas and virgaurea 

 should be compared with these. 



There is, however, still in my mind an open point as 

 regards several of the yellow species in the last group ; for 

 it is not clear by any means that we may not have in- 

 cluded here one or two physical yellows as well. We 

 know that the yellows of the first three groups are entirely 

 pigmental, for their solubility shows this ; and we know 

 that in the fourth group several species, such as Deiopeia 

 bella, C. hera lutescens, A. villica, are pigmented, since 

 their relations to red species which yield a similarly in- 

 soluble yellow proves this ; but in the case of, for instance, 

 r. pronuba (the " Yellow Underwing") and its miniature 

 analogue Heliaca, we are totally in the dark; and it appears 

 safer to me to withhold for the present any opinion as to 

 whether these be physical or pigmental yellows. Were 

 there any red underwing in the sane genus as either of 

 these, that would be suthcient to justify us, by analogy, 

 in considering the yellow of these species pigmental, just 

 as we do that of, e.g., Arctia idllica ; but failing such evi- 

 dence, the experimental evidence is not decisive in either 

 direction.-^ There is, however, a most remarkable and 

 exceptional set of phenomena connected with these yellows 

 that I once thought might prove the criterion by which 

 to distinguish between pigmental and possibly physical yel- 

 lows in doubtful cases such as that of T. pronuba. Some 

 time ago it W3s incidentally observed by Mr. Edwards 

 that the wing of a species of Colias left in a damp cyanide 

 bottle was turned red. This statement was brought under 

 my notice by Mr. T. D. A. Cockerell, first of all in the 

 columns of the Entomologist, and later in a private com- 

 munication. I must frankly admit that for a long time I 

 remained entirely incredulous of this alleged fact, since it 

 was utterly opposed to all my own experience. I had 

 observed nothing but r^/rci^r<?,yi-/7/^ modifications of colour, 

 whether by solution or simple change, and had found 

 potassic cyanide (in solution) to rapidly dissolve the 

 yellow of Colias, leaving a simple white wing : it was 

 therefore very difficult to credit such a statement. 



I will not trouble the readers of Nature with any de- 

 tailed account of my experiments^ in this direction, made 

 with the purpose of verifying — or otherwise — the correct- 

 ness of Mr. Edwardi's statement ; but will simply say that 

 I finally succeeded (owing really to a lucky accident) in 

 verifying this. A yellow wing of Colias placed on 

 W(?/ cyanide is turned red, in spite of the solvent action 

 of the cyanide : such an effect could never be attained 

 by using a cyanide solution, because all the yellow 

 would be dissolved out of the wing in a very short 

 time ; it is therefore necessary to hit the happy medium 

 between dry cyanide and solution : as it is, a good deal of 

 the yellow always goes into solution, but sufficient is left 

 in the wing to be reddened. I have not stopped, how- 

 ever, at Colias, but have examined a number of other 

 yellow species, with the result that I find many yellows 

 become changed by this method to a really brilliant red 



1 See a full discussion in the Entojitologist for January last. 



2 It is, however, to be noted that in one case I found the yellow of T. 

 pronuba very faded ; but I do not care to contend for a pigment on the 

 strength of this alone. 



3 An account of these will be found in the Entomologist for July 1891. 

 I hope that I have made it sufficiently clear that I have no shadow of a 

 claini to any credit in discovering this extraordinary phenomen.n. Most 

 certainly I should not have found it out in the course of my own experiments, 

 or even afterwards but for Mr. Cockerell's insistence on the accuracy of his 

 statement. 



