yS Colouring of the Shells of Birds' Eggs. [January, 
other, fight against each other, or unite in the incessant 
maintenance and the development of universal life ; the 
universe is transfigured to our minds ; suns succeed to 
suns, worlds to worlds, universes to universes; formidable 
movements carry all these systems through the boundless 
reg ; ons of immensity ; and everywhere to the farthest boun- 
dary where fatigued imagination can rest its wings, every- 
where developing in its infinite variety, we meet with divine 
creation, of which our microscopical planet is only an imper- 
ceptible province. 
V. ON THE COLOURING OF THE SHELLS OF 
BIRDS’ EGGS. 
By H. C. Sorby, F.R.S., &c., Pres. R.M.S. 
NE of the interesting resultsdueto the spe(5trum method 
of research, as appliedto various biological subjects, is 
that there is far greater unity and simplicity in the ele- 
mentary colouring matters than might have been expected 
from the very various tints of the natural objects. Their num- 
ber is indeed very considerable, and in some cases, though the 
colour may be nearly the same, the substances may be most 
materially different. Very often, however, we meet with an 
almost endless variety of tints, due not to any essential differ- 
ence in the elementary colouring matters, but to the varying 
proportions of a few well-marked substances. This is very 
well illustrated by the colour of many eggs — even, perhaps, 
of the same species of bird — which may, for example, pass 
from greenish through all shades of brown to reddish, brought 
about by the varying development of a blue and of a red 
constituent ; or may, in other eggs, pass from blue to very 
yellow green, owing to a variation in the relative amounts 
of a blue and of a yellow colouring matter. 
Another interesting fact connected with this subject is 
that there is often a close and remarkable relation between 
different classes of objedts and their characteristic coloured 
constituents. This is strikingly the case with the more 
important of those met with in the shells of birds’ eggs, 
since, so far, I have never met with them in any other natural 
products. Further research may, indeed, lead to their dis- 
covery elsewhere ; but, at all events, birds’ feathers of very 
similar colour contain entirely different substances. Much 
remains to be done before the subject can be looked upon 
