EVOLUTION OF INSTINCTIVE BEHAVIOUR 111 
And so, too, in many other examples of instinctive behaviour,. 
we infer from the observed facts that stimulus and response 
have an organic connection founded on hereditary links in the 
nervous system. Now, if such connection were due to inherited 
habit, we should expect them to be established wherever the 
experience to which they are related has been constant through 
many generations. How comes it, then, that the chick does 
not instinctively respond by appropriate behaviour to the 
sight of water? How comes it that young birds do not 
instinctively avoid bees, and wasps, and nauseous caterpillars ? 
If the effects of ancestral experience be hereditary, one would 
surely expect that in these cases the connection between 
stimulus and response—a connection which passes into acquired 
habit—would have become congenital; that the habitual 
behaviour would have long ago become instinctive. But this 
does not appear to be the case. And with regard to disuse 
causing the loss of instinct, how comes it that young chicks 
swim with well-ordered leg-movements, though swimming is 
not an act that is habitually performed by the members of 
their race ? : 
What, then, has the alternative hypothesis of natural 
selection to advance in explanation of these facts? On this 
hypothesis instinctive acts have biological value in such degree 
that they have become congenital through the preservation of 
adaptive variations. But if this be so, why does not the chick 
respond instinctively to the sight of that which is so essential 
to its existence as water to drink? In reply to this question 
it may be suggested that, under natural conditions, the hen 
teaches all her chickens to peck at the water, and thus shields 
them from the eliminating influence which gives rise to natural 
selection, in the absence of which the habit of drinking in 
response to the sight of water, though acquired by each 
succeeding generation of birds, has not become instinctive and 
congenital. Or, to put the matter from a slightly different 
point of view, the maternal instincts of the hen protect her 
chicks from any climination in this respect ; and in the absence 
of such elimination the habit has not been inherited as instinct. 
