79 
1876.] Colouring of the Shells of Birds' Eggs. 
as fully understood ; but, still, I have examined sufficiently 
numerous examples to be able to form a tolerably good 
general opinion. The detail would require much further 
research. A paper on the subject, published in the “ Pro- 
ceedings of the Zoological Society of London,” May 4, 1875, 
p. 351, gave a full account of what I had been able to learn up 
to that date, and since then my attention has been directed to 
entirely different subjects, so that I am unable to add much 
to what I then said, and can merely speak with greater con- 
fidence respecting some closely allied subjects. On the 
whole, I cannot do better than make some extracts from 
this paper, with any farther illustrations that may appear 
desirable. 
Hitherto I have been able to distinguish eight well-marked 
substances. One of these is identical with a colouring- 
matter met with in nearly all groups of plants, from the 
lowest to the highest ; another is probably the black pigment 
found in feathers ; but I have not yet been able to identify 
any of the rest with any found elsewhere. But, at the same 
time, I must admit that our knowledge of animal colouring- 
matters is far too limited to make such negative evidence of 
much value. All these seven coloured substances found in 
the shells of birds' eggs are insoluble in water, but soluble 
in absolute alcohol, either when neutral or when a small 
amount of free acid is present. They are also sometimes 
soluble in chloroform or carbon bisulphide. Neutral or acid 
absolute alcohol, however, is in every respeCt the most con- 
venient and best solvent. Some are extremely permanent, 
and resist the aCtion of powerful reagents, whereas others 
are of such unstable character that they are not only rapidly 
changed by acids or oxidising reagents, but are even partially 
decomposed by evaporating their solutions to a dryness at 
a gentle heat. In these general peculiarities they resemble 
bile-pigments more than any other group of colouring- 
matters, but do not actually agree with any that have passed 
under my notice. Some of them furnish us with a number 
of most interesting faCts in illustration of the probable exist- 
ence of a connection between optical properties and chemical 
or molecular constitution ; but on the present occasion I 
forbear to enter into such questions, and will confine myself 
as much as possible to the zoological aspeCt of the subject. 
At the same time, it is absolutely necessary to enter into a 
certain amount of chemical and optical details, since other- 
wise the characteristic peculiarities of the different substances 
could not be established. 
Some important and reliable information may be learned 
