Oct. 1886.] 
AND OOLOGIST. 
151 



chiefly exhibited on eggs which are deposited on 
or near the ground, as the waders’, gallinaceous 
birds’, Larks’, &c. Hewitson states: “We should 
scarcely expect to find the eggs of the Crane so 
entirely different from those of all the other 
species which are most nearly allied to it in habit 
and in form. Whilst the eggs of all these species, 
with the exception of those of the Spoon-bill, are 
either pure white or slightly tinted with colour, 
but always spotless, those of the Crane are, on the 
contrary, richly coloured.” This difference may 
be completely accounted for by the above theory, 
as the Crane habitually breeds on the ground, 
whilst the others choose elevated sites. Compare 
also the eggs of the three species of divers, laid on 
the margin of the freshwater lochs in the north, 
with those of their congeners the Guillemots, de- 
posited on cliffs. 
The coloration of the eggs of the Falconidz are 
in some respects exceptional, depending on the 
nature of the food and other causes, which lie be- 
yond the scope of the present paper. 
When I examined eggs with reference to the 
amount of light to which they were exposed, I 
found that the ratio between the intensity of the 
pigmentation and the degree of exposure was very 
marked, and, indeed, almost startling. For ex- 
ample, the eggs of birds which breed early—as 
the Thrush tribe, Hedge Sparrow, &c.—have 
weli-developed ground tints; whilst those of the 
later breeders—as the Green Finch, Linnet, &c.— 
laid after the leaves are out, and therefore 
screened from direct sunlight are more faintly 
coloured. My own experience is that exposure or 
shelter from the sun’s rays plays an important part 
in the selection of a sight for nidification, by the 
parent bird. 
It may be laid down as an almost univeral law 
in ornithology, that eggs which are deposited in 
situations exposed to the sun’s rays are much 
darker in colour than those laid in nests protected 
from direct sunlight. They almost invariably 
possess a well-developed ground tint. Some, e. 7., 
the Heron and the Hedge Sparrow, are spotless, 
but most show markings of some kind. 
Eggs laid in shaded nests, ¢. g., the Yellow- 
hammer and the Green Finch, possess a faint 
ground tint, and the markings are usually smaller 
and lighter than in the preceding. 
Eggs laid in covered nests, as the Tits’ and 
Wrens’, usually present faint spots in a white 
ground. 
Eggs which are wholly excluded from light, as 
Woodpeckers’ and Kingfishers’, are almost invar- 
iably pure white. 
Not only does direct sunlight seem necessary 
for the pigment layer to acquire its full develop- 


ment, but there is also strong evidence that all 
white and faintly coloured eggs have undergone 
or are undergoing a process of decolorization 
when the protection afforded by the pigment 
layer is no longer required. This is in accord- 
ance with the laws of physiology. If any tissue 
or organ loses its function it will gradually waste 
and finally disappear. Moreover it has been 
shown by evolutionists that the lost organ is apt 
to appear as a variation, or as a rudimentary and 
useless appendage; many eggs, therefore, show 
rudimentary pigmentation, and colour appears 
sometimes as a variation in eggs which are nor- 
mally white. Eggs have been divided by Wallace 
into white and coloured, spotted and unspotted ; 
there is, however, an unbroken series between 
white eggs and those which are highly pigmented 
and spotted. Further, the eggs of several species 
would have to be included at one time under the 
former category, and at another under the latter. 
For instance, the egg of the White-tailed Eagle is 
usually white, but occasionally it presents well- 
defined markings. The egg of the Puffin is as 
often colourless as it is pigmented. That of the 
Whinchat is of a bright blue-green, sometimes 
spotless, sometimes faintly speckled at the large 
end with rust colour. 
It has been stated that white and light coloured 
eggs are found in dark and sheltered situations 
because colour is not necessary to conceal them 
from observation. The fact has, however, been 
overlooked, that eggs laid in elevated sites, as 
Gulls’, Rooks,’ Herons’, &c., have well-developed 
colours, although they can be of no use for con- 
cealment. Want of the stimulus of the sun’s rays 
alone causes the colour to fade or disappear. 
When one (or one or two) species in a family, 
where the majority lay deeply pigmented eggs, 
lays either a white egg or one faintly coloured, 
we invariably find that it differs from the others 
in its mode of nidification, depositing its eggs in 
some place protected from the light, whilst its re- 
lations Jay in exposed situations. In this case 
there is what may be termed a specific decoloriza- 
tion, the loss of pigment affecting the individual 
species alone, and not extending to the other 
members of the family. 
When the whole of a genus or family lay white 
eggs, we find that either all, or the majority of 
the species, deposit their eggs in places protected 
from the light. In this case there appears to have 
been a generic decolorization affecting the group 
as a whole, the pigment having become obsolete 
at a much earlier period than in the preceding 
case, probably before the differentiation of the 
family into the existing number of species. 
(To be Concluded.) 
