XII INSTINCT AND REASON 337 
tether will allow him to go. But within his narrow 
limits he still continues to gain by experience. It is 
generally said that one bird builds a nest just as 
another of the same species does, hence intelligence 
cannot come into play at all. It is probable, though, 
that there are differences which escape our notice ; at 
any rate, as I have shown, individuals are capable of 
adapting themselves to new circumstances, and some 
authorities hold that a bird’s first nest is decidedly 
inferior to her later ones. 
We conclude, then, that nest building is instinctive 
but that intelligence to some extent works up- 
on and modifies the instinct. It is no argument 
against this that birds in captivity often build a very 
poor nest or are incapable of building one at all. 
Among domesticated animals instincts are apt to go 
wrong. There is a breed of hens that never sit upon 
their eggs. Among the lowest class in our big towns 
unnatural conditions of life not unfrequently lead to 
the decay of an instinct on which the continuance of 
the race depends, the affection of mother for child, an 
instinct which is never deficient in savage races. 
The Cuckoo Instinct. 
The habits of the Cuckoo are so marvellous that if 
we were to come fresh to the subject, we should be lost 
in astonishment at them. But, as Lucretius says, 
even the sun ceases in time to be an object of wonder. 
The Cuckoo lays many eggs, and we can hardly be 
wrong in seeing a connection between this fact and 
the parasitic habit. They are laid, some ornithologists 
Z 
