ed to are three: first, that the common cuckoo,
with rare exceptions, lays only one egg in a nest, so that the large and
voracious young bird receives ample food. Secondly, that the eggs are
remarkably small, not exceeding those of the skylark--a bird about
one-fourth as large as the cuckoo. That the small size of the egg is a
real case of adaptation we may infer from the fact of the mon-parasitic
American cuckoo laying full-sized eggs. Thirdly, that the young cuckoo,
soon after birth, has the instinct, the strength and a properly shaped
back for ejecting its foster-brothers, which then perish from cold and
hunger. This has been boldly called a beneficent arrangement, in
order that the young cuckoo may get sufficient food, and that its
foster-brothers may perish before they had acquired much feeling!

Turning now to the Australian species: though these birds generally lay
only one egg in a nest, it is not rare to find two and even three eggs
in the same nest. In the bronze cuckoo the eggs vary greatly in size,
from eight to ten lines in length. Now, if it had been of an advantage
to this species to have laid eggs even smaller than those now laid, so
as to have deceived certain foster-parents, or, as is more probable, to
have been hatched within a shorter period (for it is asserted that
there is a relation between the size of eggs and the period of their
incubation), then there is no difficulty in believing that a race or
species might have been formed which would have laid smaller and smaller
eggs; for these would have been more safely hatched and reared. Mr.
Ramsay remarks that two of the Australian cuckoos, when they lay their
eggs in an open nest, manifest a decided preference for nests containing
eggs similar in colour to their own. The European species apparently
manifests some tendency towards a similar instinct, but not rarely
departs from it, as is shown by her laying her dull and pale-coloured
eggs in the nest of the hedge-warbler with bright greenish-blue eggs.
Had our cuckoo invariably displayed the above instinct, it would
assuredly have been added to those which it is assumed must all have
been acquired together. The eggs of the Australian bronze cuckoo vary,
according to Mr. Ramsay, to an extraordinary degree in colour; so
that in this respect, as well as in size, natural selection might have
secured and fixed any advantageous variation.

In the case of the European cuckoo, the offspring of the foster-pare