50 
The Action of Light upon the 
[January, 
separated, as in Papilio Machaon. We have, further, to 
throw a light upon the origin of the pigment-reds, to two of 
which Mr. Wallace refers as being different in their chemical 
constitution and behaviour. 
But chlorophyll is not the only substance which has been 
called into requisition in order to explain the mysteries of 
animal colouration. It has not escaped the attention of 
biologists that all those creatures which develope, more or 
less frequently, beautiful hues are precisely the same in 
which uric acid is abundantly secreted, — i.e., birds, reptiles, 
inserts, — whilst in the Mammalia, in which the secretion of 
uric acid is trifling, the prevailing colours are dull. It was 
asserted that whilst uric acid is abundantly found in the 
excretions of parrots, humming-birds, &c., at other times of 
the year, during and immediately before the moulting season 
it was absent. Hence the inference that this compound 
might play a part in the elaboration of the new plumage 
was not unwarrantable. In addition came the fadt that a 
beautiful violet colour, known as murexide, and capable of 
producing a variety of shades, was artificially obtained from 
uric acid.* Unfortunately when these investigations were 
carried on the distinction between interference-colours and 
absorption-colours had not been fully apprehended, and the 
iridescent hues of humming-birds, trogons, Belionotce , were 
supposed to be due to some peculiar pigments of unknown 
composition. Nor has it, as far as we are aware, ever been 
shown that the excreta of splendidly-coloured birds are 
richer in uric acid than those of sea-fowl. For the present, 
therefore, the uric acid theory must be considered as useless. 
A consideration of the food of different species might at 
first sight appear likely to throw some light upon the nature 
of their colouration. But we find intense splendour and 
varied tints alike among carnivorous species (Cicindelidse 
and certain Carabidae), wood-eaters (Buprestidae), and leaf- 
eaters (Chrysomelidae). We find dull and sordid colours 
among many carrion- and dung-feeders (Silphidae, Aphodiidae, 
Staphylinidae), whilst others addicted to a similar diet — 
such as most species of the genus Phanceus — display the 
most splendid hues. Nor is an examination of the diet of 
birds more satisfactory. t 
* Murexide, known in the commercial world as “Roman purple” and 
“ Tyrian purple,” was some time ago prepared from guano, — i.e., the excreta 
of sea-fowl, — and was in considerable demand among dyers and calico-printers. 
Being costlier than the coal-tar colours, it is now superseded. 
f In addition to the case cf chlorophyll above mentioned there seem to be 
individual instances where the colouring matter of a plant, if eaten by insects, 
may be traced in their secretions. We do not know whether the deep reddish 
