88 
Colouring of the Shells of Birds' Eggs. [January, 
species or in the more or less variable eggs of the same kind 
of bird, or in patches on the same egg, can thus be explained 
without any difficulty. 
In a similar manner the various shades of green, passing 
from the blue-green of such eggs as those of the common 
Hedge Sparrow to the fine malachite green of the fresh 
Emu, and to the very yellow-green seen on them in patches, 
are all due to a variable mixture of oocyan with yellow 
ooxanthine. 
As is no doubt well known, many green eggs turn blue 
on long keeping. In this manner the beautiful malachite 
green of fresh Emu-eggs passes into dark blue. This is 
easily explained by the fact that yellow ooxanthine is much 
more easily destroyed by oxidisation than oocyan. A portion 
of a green Emu egg exposed to strong light soon becomes 
much bluer, and so does a mixed solution of the two colour- 
ing-matters in alcohol, the yellow constituent being destroyed 
and the blue left. 
A few eggs are of a brick-red colour. Those of Cetti’s 
Warbler are as good an example as any I have seen ; and on 
carefully comparing them with the browner red egg of the 
common Grouse, I found that both contained a large amount 
of oorhodeine, but that the tint was made more dull in the 
case of the Grouse by the presence of a small quantity of 
the colouring-matter which gives the narrow bands in the 
red ; whereas in the case of Cetti’s Warbler this was almost 
or quite absent, and there was present a relatively very 
unusually large amount of the orange-coloured substance, 
which I have not been able to distinguish from lichno- 
xanthine. To the presence of this substance we may thus 
attribute the brick-red tints seen in a few eggs. 
The eggs of the black variety of the common Duck are 
coloured with a nearly black substance, which I have not 
yet obtained in a state of solution, and probably corresponds 
to the so-called pigmentum nigrum, which may be obtained 
in larger quantity in a more satisfactory condition from 
black hairs or feathers, and used for drawings as a splendid 
black pigment, quite equal to the best Indian ink. 
My studies of colouring-matters by the spectrum method 
soon led me to perceive that the individual species of certain 
groups of coloured substances are so intimately connected 
with their life that plants may be arranged in a kind of 
natural order according to the presence, absence, or relative 
proportion of the various coloured constituents, which order 
on the whole agrees remarkably with that founded on struc- 
tural characters, as shown in my paper on comparative 
