1876.] Colouring of the Shells of Birds' Eggs . gi 
the Tinamous appears to be equally well distinguished by 
the formation of a special colouring-matter in the egg. 
Connection between the Colouring-matters of Eggs and other 
Organic Products . 
It would obviously be very interesting to learn what con- 
nexion 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 and the pigmentum nigrum, I have not been 
able to identify them with confidence except in the shells of 
birds’ eggs. The speXra of oorhodeine are so well marked 
that there could be no difficulty in recognising a compara- 
tively small quantity; and yet no trace can be deteXed in 
feathers whose general colour is praXically 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 produXs, we 
are at once brought face to face with a branch of inquiry 
which seems to promise 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 distinXly 
different compounds these may and do generally give speXra 
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 speXra. We may 
perhaps better understand the faXs by supposing that when 
a substance combined with the coloured constituent is 
replaced by some other, the general shape and constitution 
of the ultimate molecules is so slightly changed that the 
general charaXer of the speXrum is the same, but the size 
of the molecules 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 exaXly the same speXra and are changed in the 
same manner by the addition of reagents — in faX 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 probable 
that there is some important chemical or physical relation 
between the two compounds. 
* Monthly Microscopical Journal, vol. xiii. p. 198. 
