112 INSTINCTIVE BEHAVIOUR 
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, 
without doing much to determine, survival. In any case these 
little instinctive traits, if they are so trivial as to seem of small 
value from the biological point of view, appear to be too 
unimportant to have been intelligently acquired as habits. 
Let us now consider one or two cases of instinctive behaviour 
which would fall under Romanes’s category of instincts of 
blended origin partly due to natural selection, partly to the 
inheritance of acquired habit. It is the custom of the house 
martin to build beneath the eaves. Forsaking the ancestral 
rocky haunts, it has been led to utilize the houses that 
man has built. This has all the appearance of being due 
to an intelligent modification of the ancestral instinct ; but 
how far the modification has become through heredity a 
