Oct. 1886.] 
AND OOLOGIST. 
149 


fertilization, and most sea birds are white, with 
blue on the back, resembling the colours of the 
clouds, and rendering them less likely to frighten 
the fish upon which they prey. 
Much has been written about the variety of 
colours observed on eggs, but a glance at the 
solar spectrum will at once show the fallacy of 
this opinion. The only portions of the spectrum 
represented on the whole range of colours of 
birds’ eggs are a small band on the red (forty to 
forty-five), and another on the green (eighty-five 
to ninety-two). . 
This seems the more striking when we consider 
the number of colours exhibited by the birds 
themselves, where every part of the spectrum is 
represented, as well as by other organic objects, 
as insects, flowers, &c. Only two colours of pig- 
ment are found on eggs, green and red—together 
with black, which is not a colour in the scientific 
sense of the term. It is a remarkable fact, and 
one which I have never seen noticed by writers 
on the subject, that all the rich hues and shades 
with which eggs are so lavishly ornamented, are 
merely intermediates between these three. 
I have arranged the following scale by an ex- 
pansion of these three according to chromatic 
rules, and it will be seen that it embraces every 
shade of colour found in bird’s’ eggs. 
BLUE-GREEN. Russet or Orange Brown. 
Green. Brown. 
Olive-Green. Grey-Brown. 
Olive. Brown-Grey. 
Olive-Brown. Black or Grey. 
The hematin of the red blood-corpuscles is the 
source of these pigments. It is taken up by the 
pigment cells where it undergoes metamorphosis 
into the green, brown and black shades, and is 
then secreted by the particular follicles at the 
lower part of the uterus. “To understand how 
yellow, green, brown and black pigments may be 
derived from the colouring matter of the blood,” 
writes Rindfleisch, ‘‘we must first glance at the 
physiological metamorphoses to which this sub- 
stance is liable. The most important of these, and 
in some sense typical of all the rest, is its trans- 
formation into bile-pigment. The red corpuscles, 
as they grow old, part with their colouring matter 
to the serum; from this it is taken up by the liver- 
cells, which transform it into bile-pigment; as 
such it is ultimately excreted in the feces. Before 
it is thus removed, when retained in the gall 
bladder for any length of time, it undergoes 
further changes, passing through shades of yellow, 
green, brown and black, which Shiideler terms 
respectively bilifuscin (Cs2H2N2Og), biliverdin 
(Cs2H20N2010), biliprasin (Cs2H22N2O012), and bili- 
humin; bilifuscin differing from bilirubin in con- 
taining two atoms more HO, biliverdin from bili- 
fuscin in containing two atoms more O, biliprasin 
from biliverdin, again by an access of 2HO, while 
bilihumin is a black, insoluble, very highly oxi- 
dised substance.” 
‘“‘The scale of colours enumerated above 
serves, as already stated, as a standard for the 
course of all other chromatases, whether physiolo- 
gical or pathological.” 
A careful study of these pigments will show 
the reason for this limited range of colour. If we 
examine the characteristics of the pigment layer 
on eggs, it is impossible to resist the conclusion 
that its presence has reference to the sun’s rays. 
All organic objects which are liable to be exposed 
to the sun’s rays are protected by pigment of one 
colour or another. This is the chief use of pig- 
ment in the animal world. In many animals the 
colours are adapted or modified for concealment, 
but their primary use is for protection from the 
glare of the sun. The delicate and tender ovum 
in particular requires protection. Professor Yung 
has shown the generating power of the sun’s rays 
on the ova of frogs In 1881, I read a paper 
showing that the pigment of frog’s spawn was 
placed in the ovum itself to absorb the sun’s rays 
as well as to protect the organism*. But the 
ovum of the more highly organised warm- 
blooded animals does not derive the stimulus re- 
quired for its development from the rays of the 
sun but from the body of the parent. I think that 
this affords us sufficient reason for assuming that 
the use of the pigmented covering of the shell is 
to protect the sensitive ovum from being acted on 
by the sun’s rays. If we make two perforations 
in a piece of cardboard, cover one with a piece of 
pigmented egg shell (¢. g., a rook’s or blackbird’s), 
and the other with a white one (e. g., a pigeon’s or 
a woodpecker’s), and hold the cardboard up to 
the light, we see the great amount of protection 
afforded by the pigmented shell. ‘True, the same 
end might be gained by thickening the shell, but 
it would have to be increased several fold to af- 
ford an equal amount of protection, and nature is 
never wasteful in material. Moreover, owing to 
the peculiar way in which the yolk is suspended 
by the chalaze, the germinal spot is always up- 
permost, and consequently exposed to the sun’s 
rays striking from above. 
Mr. Salvin noticed in Guatemala that Hum- 
ming-birds were much more unwilling to leave 
their nests during very hot weather, when the 
sun was shining brightly, than during, cool, 
cloudy, or rainy weather. 
I shall afterwards show that there is a direct 
relation between the amount of light to which the 
