1876.] Colouring of the Shells of Birds' Eggs. 87 
reason to suspedt that many yellow feathers are coloured by 
the far less stable xanthophylls derived from that source. 
The loose yellow pigment on the beak of the hornbill, with 
which he paints his feathers and makes himself look a far 
smarter bird than he otherwise would be, corresponds in 
every particular with a mixture of xanthophyll with lichno- 
xanthine and a little yellow xanthophyll, all of which could 
be so readily absorbed from the vegetable food along with 
fatty or waxy substances, that it seems unnecessary to sup- 
pose that they are formed in the bird’s own organism, though 
this is of course quite possible. 
Such, then, is a general account of those peculiarities of 
the colouring matters that have come under my notice, 
which suffice to distinguish them from one another and from 
analogous substances met with elsewhere ; and I now pro- 
ceed to a more detailed consideration of the eggs themselves. 
As an illustration of the method of study, suppose that we 
have taken portions of the brownish-red eggs of the common 
Grouse, of the pure brown eggs of the Nightingale, and of 
the pure blue of the common Thrush, separated from the 
black spots, kept for examination by themselves. After 
having, in each case, dissolved out the carbonate of lime with 
dilute hydrochloric acid and having washed the residues with 
water, they should each be digested in cold neutral absolute 
alcohol. Scarcely any colour would be dissolved out in 
the case of the Grouse — but a fine blue in all the others, 
which, on further examination, would be found to be oocyan, 
with mere traces of other substances. After having dissolved 
out as much as possible, by means of fresh neutral alcohol, 
the residue should be digested in alcohol with a small 
quantity of hydrochloric acid. It would then be found 
that the Grouse-shell would give a rose-coloured solution, 
containing much of the acid modification of oorhodeine. 
The Nightingale would also give much oorhodeine, but 
the colour would be modified by the presence of oocyan ; 
the blue portion of the Thrush-egg would give a small 
quantity of a fine blue substance, showing the spedtrum of 
banded oocyan, with little or no trace of oorhodeine, whereas 
the dark spots would be found to give a very considerable 
quantity of oorhodeine. We thus clearly see that the redder 
egg is mainly coloured with oorhodeine ; the blue egg with 
oocyan — the brown colour of the Nightingale being due to a 
mixture of these two, and the black spots on the Thrush-egg 
to patches containing much oorhodeine. All the various 
intermediate shades of colour, passing from red through 
brown to blue, whether they occur in the eggs of different 
