112 INSTINCTIVE BEHAVIOUR 
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 would be 
eliminated which, on pecking the water, failed to respond to 
the stimulus by the complex behaviour involved in drinking, 
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 
with the stimulus of water in the bill. Whence we may infer 
that the co-operation of natural selection is an essential factor 
in the evolution of instinctive behaviour. 
There are, however, cases of instinctive behaviour which 
may seem too trivial and unimportant to be subject to the 
sway of natural selection. ‘There are numberless little idio- 
syncracies of behaviour which seem to be truly instinctive, 
which are readily recognizable as distinctive traits, but which 
can hardly be regarded as of sufficient biological value to 
determine whether the creatures in which they are developed 
should survive or be eliminated in the struggle for existence. 
In many cases, however, these serve rather to distinguish the 
detailed manner of behaviour than its biological end or 
purpose. In different species natural selection may determine 
the survival of those whose instinctive behaviour meets a 
biological need. The relatively unimportant details, differing 
slightly in each species, are mere adjuncts ; and since natural 
selection deals with each species or inter-generating group 
separately, the essential behaviour may in each case carry with 
it the associated differences of manner. We must remember, 
too, that, as in the matter of structure so in that of behaviour, 
it is the animal as a whole that is selected for survival ; and 
so long as the whole is adapted to the circumstances of life, 
the associated differences of form or manner may share in, 
