362 MR. H. C. SORBY ON THE [May 4, 
seen any facts which seem to indicate that it occurs in any other 
group of birds than the Tinamous, unless, indeed, it be in the case of 
the eggs of Caswarius bennettii and C. australis. If further exa- 
mination should confirm these conclusions, it appears to me that the 
facts will be of much interest in connexion with comparative physio- 
logy, as showing that, to a limited extent, even in the case of birds’ 
eggs, there is a connexion between the general organization of the 
animals and their coloured secretions, since, as will be seen, such a 
well-marked group of birds as the Tinamous appears to be equally 
well distinguished by the formation of a special colouring-matter in 
the egg. 
CONNEXION BETWEEN THE COLOURING-MATTERS OF EGGS 
AND OTHER ORGANIC PRODUCTS. 
It would obviously be very interesting to learn what connexion 
there is between the various colouring-matters described in this 
paper and substances met with elsewhere. Perhaps further inquiry 
may lead to the discovery of some of them in other situations ; but, 
with the exception of lichnoxanthine, I have not yet detected or been 
at all events able to identify them with confidence except in the 
shells of birds’ eggs. The spectra of oorhodeine are so well marked 
that there could be no difficulty in recognizing a comparatively small 
quantity ; and yet no trace can be detected in feathers whose general 
colour is practically identical with that of birds’ eggs coloured with 
oorhodeine. 
In considering the relation between the coloured substances in 
birds’ eggs and other natural or artificial products, we are at once 
brought face to face with a branch of inquiry which seems to pro- 
mise most valuable results, but is now so much in its infancy 
that the conclusions can only be looked upon as very plausible. In 
a paper recently read before the Royal Microscopical Society* I have 
shown that in some cases it is certain, and in others probable, that 
when a coloured constituent is common to a number of distinctly 
different compounds these may and do generally give spectra which 
are most intimately related in the ratio of the wave-lengths of the 
centres of their absorption-bands, but the actual wave-lengths differ 
in the different spectra. We may perhaps better understand the 
facts by supposing that when a substance combined with the coloured 
constituent is replaced by some other, the general shape and consti- 
tution of the ultimate molecules is so slightly changed that the 
general character of the spectrum is the same, but the size of the 
molcules so far altered that they are put into relation with waves 
of light of a different length. It appears to me therefore that, 
when we meet with two substances which give almost exactly the 
same spectra and are changed in the same manner by the addition 
of reagents—in fact differ from one another only in the numerical 
values, and not in the relations of the wave-lengths of the bands or 
in any other essential particular—we may look upon it as very 
* Monthly Microscopical Journal, yol. xiii. p. 198. 
