nize sufficiently with its
surroundings to escape observation from prowling egg-stealers of all
kinds.

The size of the egg depends partly on the number produced and partly on
the conditions determining the state of the young bird at hatching:
hence there is a great disparity in the relative sizes of the eggs of
different birds. Thus it will be found that young birds which emerge in
the world blind, naked and helpless are the product of relatively small
eggs, while on the contrary young hatched from relatively large eggs are
down-clad and active from birth.

The fact that the eggs must be brooded by the parent is also a
controlling factor in so far as number is concerned, for no more can be
hatched than can be covered by the sitting bird. Other factors, however,
less understood, also exercise a controlling influence in this matter.
Thus the ostrich lays from 12 to 16, the teal 15, the partridge 12-20,
while among many other species the number is strictly limited, as in the
case of the hornbills and guillemots, which lay but a single egg; the
apteryx, divers, petrels and pigeons never lay more than 2, while the
gulls and plovers never exceed 4. Tropical species are said to lay fewer
eggs than their representatives in temperate regions, and further
immature birds lay more and smaller eggs than when fully adult.

Partly owing to the uniformity of shape, size and texture of the shell,
the eggs of birds are by no means easy to distinguish, except in so far
as their family resemblances are concerned: that is to say, except in
particular cases, they cannot be specifically distinguished, and hence
they are of but little or no value for the purposes of classification.

Save only among the megapodes, all birds brood their eggs, the period of
incubation varying from 13 days, as in small passerine birds, to 8
weeks, as in the cassowary, though eggs of the rhea and of _Struthio_
hatch in from 5 to 6 weeks. But the megapodes deposit their eggs in
mounds of decaying vegetable matter or in sand in the neighbourhood of
hot springs, and there without further apparent care leave them. Where
the nestling is active from the moment of hatching the eggs have a
relatively longer incubation period than in cases where the nestlings
are for a long while helpless.

_Eggs of Mammals._--Only in the spiny ant-eater, or _Echidna_, and the
duck-billed platypus, or _Ornithorhynchus_, among the Mammalia, are the
eggs provided with a large store of