Aug., 1892. colours and coloration in animals. 
17B 
Consequently a study of the origin and meaning of animal 
pigments is of the highest importance to a correct under¬ 
standing of the problems of coloration. At present very 
little is known about these important factors in the question ; 
they may be divided into four classes : — 
(1.) Pigments of direct physiological importance, as for 
instance tetronerythrin, common in sponges, and haemo¬ 
globin, which are both respiratory in function. 
(2.) Pigments which are waste products stored up in the 
skin ; such is the yellow pigment present in the scales of the 
Brimstone, Clouded Yellow, White, and other butterflies; it 
is a compound related to uric acid. 
(8.) Pigments obtained from other animals, and more or 
less altered ; the green colour of the blood in caterpillars is 
due to the slightly altered chlorophyll obtained from the plants 
on which they feed. It is suggested by Mr. Beddard that 
the frequent resemblance between animals which live toge¬ 
ther, as that of certain nudibranclis, annelids, and starfish 
to the corals on which they feed, may simply be due to the 
presence in the former of unaltered pigment obtained from 
the latter, so that the aid of natural selection need not be 
invoked to explain the likeness. 
(4.) The pigment, as far as known, is of no use to the 
animal except in so far as it produces a resemblance to 
external objects which benefits the owner. In such cases, 
which are many, investigation is much needed into the nature 
and origin of the pigment. 
Colour, then, depends on the physical constitution 
of pigment or of the tissues; anything which affects 
the physical constitution of the body may produce 
changes of colour. The external agents likely to be 
concerned are food, heat, cold, dryness, moisture, light, and 
darkness. There are several well authenticated instances of 
the effect of food on colour. Thus, the larvae of the Large 
Tortoise-shell butterflies when fed on nettles, which are not 
their usual food, formed imagoes showing a wonderful 
similarity to those of the Small Tortoiseshell which usually 
feed on nettles. Again, some pupae of Saturnia brought from 
Texas to Switzerland developed into normal forms ; the 
caterpillars which they produced were fed on Juglans regia, 
J. nigrans being their food plant in Texas ; the moths into 
which they turned were so different from the parent as to be 
reckoned a distinct species. 
As to climate, the effects seem considerable. Thus heat 
and moisture seem to produce a darkening of coloration, 
while cold and darkness have the opposite effect. The 
