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-parents are
commonly ejected from the nest within three days after the cuckoo is
hatched; and as the latter at this age is in a most helpless condition, Mr.
Gould was formerly inclined to believe that the act of ejection was
performed by the foster-parents themselves. But he has now received a
trustworthy account of a young cuckoo which was actually seen, while still
blind and not able even to hold up its own head, in the act of ejecting its
foster-brothers. One of these was replaced in the nest by the observer,
and was again thrown out. With respect to the means by which this strange
and odious instinct was acquired, if it were of great importance for the
young cuckoo, as is probably the case, to receive as much food as possible
soon after birth, I can see no special difficulty in its having gradually
acquired, during successive generations, the blind desire, the strength,
and structure necessary for the work of ejection; for those cuckoos which