360 MR. H. C. SORBY ON THE [May 4, 
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 com- 
paring them with the browner red eggs 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 pre- - 
sence 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 lichnoxanthine. To the pre- 
sence of this substance we may thus attribute the brick-red tints seen 
in a few eggs. 
CoNNEXION BETWEEN THE COLOURING-MATTERS OF THE 
EGGs AND THE STRUCTURE OF THE Birps. 
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 structural characters, as shown in my paper on compa- 
rative vegetal chromatology.* This naturally led me to consider 
whether any such connexion could be recognized in the case of 
birds’ eggs. Much remains to be learned before any positive opi- 
nion can be expressed; but what is already known appears to be 
sufficient to prove that, if there be any definite connexion between 
the general organization of birds and the coloured substances found 
in their eggs, it is not of such a kind as is at all obvious to any one 
who, like myself, is not thoroughly acquainted with anatomical de- 
tails. Six out of the seven different colouring-matters occur in 
variable amount in a very great variety of eggs, but there is no 
greater variation than is met with in the different individual eggs of 
the common Guillemot; so that the study of the colouring-matters 
cannot be looked upon as of any value in distinguishing species, or 
even much wider groups, except, perhaps, in one particular instance. 
Hitherto I have met with rufous ooxanthine only in the eggs of the 
Tinamous, and perhaps in those of some species of Cassowary ; and 
though the question needs further examination, it is perhaps desirable 
to give a short account of what is already known. 
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 which may correspond to the so-called pigmentum 
nigrum; but whether it is a simple substance or a mixture remains 
to be determined, and therefore it would be premature to class it 
with the other more typical colouring-matters, 
EaGeGs or THE TINAMOUS. 
As previously described, rufous ooxanthine when in solid form in 
* Proceedings of the Royal Society, xxi. p. 442. 
