ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
459 
in no case are different coloric reactions produced by the two classes of 
reagent. He next gives a list of the species experimented upon, the 
results being chiefly in tabulated form ; and then proceeds to discuss 
the significance of these results. He distinguishes between 'pigmental 
and physical colours, the latter of which he subdivides into interference 
colours, reflection colours (other than interference colours), and absorption 
colours. He then discusses the colours one by one. 
Black he finds in every instance, almost without exception, is un- 
affected by his reagents ; and after discussing the subject in detail, he 
concludes that black is merely a (physical) absorption colour, and not 
due to any pigment. He points out the surprising nature of this result, 
seeing that a black pigment exists so commonly in the animal kingdom ; 
and also remarks that, as a consequence of this, his experiments fail to 
throw any light on the melanic varieties of Lepidoptera. 
White also he finds to be no pigment colour, but simply a reflection 
effect. In one case, however ( Arga galathea ), he found the white wing 
changed to a deep yellow, which yellow finally dissolved, leaving the 
wing colourless; he explains this by supposing an unstable pigment- 
mother-substance to exist in this species; this mother-substance is 
decomposed by his reagents with the production of the yellow pigment, 
whose subsequent dissolution is comparable with the behaviour of various 
normally yellow species. 
Before discussing the other colours in detail, the author justifies his 
assumption that the changes induced by his reagents are uniformly retro- 
gressive ; and then proceeds to distinguish (among pigmental colours) 
between the “ reversion ” and the “ soluble ” effects. Some colours 
are soluble, and then the wing is permanently discoloured ; but in the 
case of red, the effect of acid is to change this instantly to yellow, 
which yellow may subsequently be restored to red, and the process 
repeated indefinitely : this is the “ reversion ” effect. 
Yellow and red he finds to be very closely related. In nearly every 
species red or pink is changed to yellow ; but the yellow thus produced 
cannot be further affected, except in the very interesting case of species 
of Delias , in which the yellow so formed subsequently dissolves, 
leaving a white wing. He distinguishes at least three stages of coloric 
evolution in yellow. In the first stage the yellow is completely soluble 
in his reagents, leaving a pure white wing ; and these yellows are very 
often of a pale tint. In the second stage, the yellow is only somewhat 
affected ; and in the third stage (which also includes all the metamor- 
phosed reds) the yellow is absolutely indifferent to the reagents : these 
last yellows are very often of a deep tint. The author proposes to 
account for all these facts on the theory of the gradual evolution of a 
deep yellow and finally of a red, from the primitive pale yellow ; but he 
specially insists that this yellow was not developed from any white 
pigment, although usually in a white wing ; experiments tending to 
prove this contention are cited. The author reserves his opinion as to 
whether among the unaffected yellows there may not be one or two 
that are not pigmental but simply “ physical ” colours. 
The “ reversion ” experiments on red are next described. The 
author finds that a red wing when yellowed by nitric acid is permanently 
yellow ; but when yellowed by other acids, the yellow is permanent 
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