306 
ANIMAL INTELLIGENCE. 
she occasionally ]aid her egg in another bird’s nest. If the old 
bird profited by this occasional habit through being able to 
migrate earlier, or through any other cause; or if the young 
were made more vigorous by advantage being taken of the mis- 
taken instinct of another species than when reared by their own 
mother, encumbered as she could hardly fail to be by having 
eggs and young at the same time ; 1 then the old birds or the 
fostered young would gain an advantage . 2 
The instinct would seem to be a very old one, for there 
are two great changes of structure in the European cuckoo 
which are manifestly correlated with the instinct. Thus, 
the shape of the young bird’s back has already been noted ; 
and not less remarkable than this is the small size of the 
egg from which the young bird is hatched. For the egg 
of the cuckoo is not any larger than that of the skylark, 
although an adult cuckoo is four times the size of an adult 
skylark. And 6 that the small size of the egg is a real 
case of adaptation (in order to deceive the small birds in 
whose nests it is laid), we may infer from the fact of the 
non-parasitic American cuckoo laying full-sized eggs.’ 
Yet, although the instinct in question is doubtless of high 
antiquity, there have been occasional instances observed 
in cuckoos of reversion to the ancestral instinct of nidifica- 
tion ; for, according to Adolf Muller, 4 the cuckoo occasion- 
1 Allusion is here made to the fact that the cuckoo lays her eggs at 
intervals of two or three days, and therefore that if all were incubated 
by the mother, they would hatch out at different times — a state of things 
which actually obtains in the case of the American cuckoo, whose 
nest contains eggs and young at the same time. 
* It is worth while to observe, as bearing on this theory of the origin 
of this parasitic habit, that even non-parasitic birds occasionally deposit 
their eggs in nests of other birds. Thus, Professor A. Newton writes in 
his admirable essay on ‘ Birds ’ in the Encyclopaedia Britannica, ‘ Cer- 
tain it is that some birds, whether by mistake or stupidity, do not un- 
frequently lay their eggs in the nests of others. It is within the know- 
ledge of many that pheasants’ eggs and partridges’ eggs are often laid 
in the same nest ; and it is within the knowledge of the writer that 
gulls’ eggs have been found in the nests of eider-ducks, and vice versa; 
that a redstart and a pied flycatcher will lay their eggs in the same con- 
venient hole — the forest being rather deficient in such accommodation ; 
that an owl and a duck will resort to the same nest-hole, set up by the 
scheming woodman for his own advantage ; and that the starling, which 
constantly dispossesses the green woodpecker, sometimes discovers that 
Lie rightful heir of the domicile has to be brought up by the intruding 
tenant.* 
