EVOLUTION OF INSTINCTIVE BEHAVIOUR 111 
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 elimination in this respect ; and in the absence 
of such elimination the habit has not been inherited as instinct. 
But though the hen can lead her young to peck at the water, 
she cannot teach them how to perform the complex move- 
ments of the mouth, throat, and head in actual drinking. In 
this matter, therefore, her own instinctive procedure does not 
shield them from the incidence of that elimination which leads 
to survival under natural selection. Those chicks which, on 
pecking the water, failed to respond to the stimulus by the 
complex behaviour involved in drinking would be eliminated, 
leaving those to survive in which the response had been 
congenitally established. Thus it would seem that, when 
natural selection is excluded, the habit has not become con- 
genitally linked with a visual stimulus; but when natural 
selection is in operation, the response has been thus linked 
