320 MR. E. B. POULTON ON THE COLOUR-RELATION BETWEEN EXPOSED 
necessary to make a standard of comparison by the selection of well-marked degrees 
of colour, for in this species there is no distinct dimorphism, but all variations are 
connected by intervening forms. At this period of the investigation I arranged the 
pup® according to the following standard, which was subsequently rendered more 
elaborate as the experiments became more precise. (Six of the most important 
varieties are figured in Plate 26, figs. 1-6, X 2.) 
(1) Very unusually dark, almost black ; very little gold, or none. 
(2) Dark normal form ; dark grey, often with a slight pinkish tinge, with very 
little gold, or none. 
(3) Light normal form ; light grey, often with a pronounced pinkish tinge ; 
more gold than (2), occasionally none. 
(4) Lighter than (3) ; the pinkish tinge often very pronounced, and usually a 
large amount of gold. 
(5) Very light forms ; generally completely covered with gold. 
It will be noticed that the dark colour due to pigment is shown by the above list 
to be developed in inverse ratio to the amount of gilded appearance, which depends 
upon a totally different optical condition, and pigment is absent from the gilt spots of 
the darkest varieties. The pink tinge forms the ground-colour of the pigment tints, 
while the darker forms are due to the increase in number of dark cloudy spots 
and the widening and multiplication of the strands of similar dark reticulations, 
which, in the most extreme forms, completely obscure the ground-colour. The 
pigment of both ground-colour and the dark spots is cuticular in position. Two 
degrees of pigmentation are shown in figs. 8-9, X 7, Plate 26, the pupal wings alone 
being represented, but affording a fair criterion of the general development of 
pigment on the pupal surface. 
In comparing the results of experiments by means of a standard, it is obvious that 
the classification of any series of pup® is, at any rate, correct as affording a test of 
the relative amount of pigment, gilding, &c., upon the pup® compared together at 
any one time. But the experiments were continued for a much longer time than 
that passed in the pupal stage by this species, and hence it is possible that the 
classification of the pup® in the later experiments will not bear too close a comparison 
with that of the earlier ones. Nevertheless, I do not think that there was much 
difference, and when the times for taking results happened to come sufficiently near 
together I always classified the pup® together, although taking notes of each 
company separately. But, however great be the want of parallelism between the 
arrangement of the initial and terminal experiments, the results would not be invali¬ 
dated, for the numbers made use of in several of the series of experiments which became 
ready for classification together were amply sufficient to ensure conclusive results, 
and for each series classified together the arrangement would certainly hold good. 
Mr. Pode’s larv® were divided into several lots, which are described as follows, 
