48 
obscured the processes and made explanation difficult, but modern 
enquiry has opened up a most fruitful series of suggestions relative to 
the results arrived at in these experiments. 
VARIATION OF COLOURS IN INSECTS. 
This leads me to bring before your consideration again, a matter 
connected with speculative entomology, which is always of the greatest 
interest to me, viz., the causes of the variation of colour in insects. 
This phenomenon occurs under a multitude of different conditions, 
and is presented to us in an infinite variety of ways. The causes of 
these variations are probably equally varied. The principle of the 
theory of mimicry by which certain species of butterflies which are 
nauseous as food to insectivorous birds, reptiles, mammals, etc., are 
mimicked by other species that are not nauseous, and which are there¬ 
fore much preyed upon by their enemies, is surely old ground to you 
all. You have all, of course, read the papers by Bates, Wallace and 
Trimen on the subject, and are conversant with the facts. Neither 
can any one, with the least claim to be considered a scientific amateur 
entomologist, have missed Dixey’s later papers on the same subject. 
I have recently brought under your notice, in a series of short articles 
in the Entomologist's Record, vol. viii., Weismann’s latest views of the 
protective resemblance exhibited by the “ leaf butterflies ” of the 
tropics. 
Now, although the value of the protective resemblance of Paralekta 
inachis and its allies to the leaf of a tree, and the advantage of the similar 
coloration of the unprotected Pierids to that of the nauseous Nympha- 
lids, were patent at once to everyone—entomologist or not—yet, 
satisfactory explanations of these phenomena were not for some time 
forthcoming. Muller and others attempted various explanations, but 
it was not until quite recently that Dixey and Weismann gave really 
scientific explanations as to the probable modus operandi by which 
these resemblances were actually brought about in the insect. 
The explanations of Dixey as to mimicked species, and those of 
Weismann as to protective resemblance, leave us, however, quite in 
the dark as to one important factor, viz., what are the physical changes 
in the organism, which have brought about the change in the colora¬ 
tion in each instance? We want to know what change the pigment- 
factor, the ornament material, as it were, has undergone, or in what 
way the scale structure has been altered to bring about these changes 
of colour. 
The difference in the colour of lepidoptera is due largely to one of 
three things, (1) a modification of scale structure, (2) a change in the 
scale-contents (pigment-factor), (3) a change in the pigment-factor of 
the basement-membrane of the wing. So far back as January, 1892, 
in the Introduction to Vol. ii. of The Bntish Noctuae and their 
Varieties, I published a number of observations on the physiological 
basis of the variation in the colours of insects, and on the genetic 
sequence of the development of these colours. Among other facts, I 
proved the possession, by certain “ white ” butterflies, of an unstable 
white pigment in their scales. In 1894, this white pigment was 
isolated and examined by Dr. F. Gowland Hopkins, and the same 
experimenter showed that, under certain conditions, the white pigment- 
factor could be readily changed to yellow. It happens that the white 
