one place in anything
so complex as an inherited disposition? Again, since we have to confess
to so very much ignorance on so many points connected with the whole
phenomenon of migration, may there not be some condition, hitherto
shrouded in mystery, which might place so different a complexion on the
corresponding aspect of migrant behaviour as to rid us, in their case
also, of the necessity of appealing to an inherited disposition? Such
questions are justifiable. And if the life-histories of other species
gave no further support to our interpretation, if, in short, the
evidence were to break down at this point, then we should be forced to
seek some other explanation more in keeping with the general body of
facts.

But far from placing any obstacle in the way of an interpretation in
terms of inherited disposition, the behaviour of many of those residents
which are not paired for life gives us even surer ground for that
belief. Moreover in their case the initial stages in the process are
more accessible to observation. I will endeavour to explain why. In the
process of reproduction the environment has its part to play--whether in
the manner here suggested, or indirectly through the question of
food-supply, matters not at the moment. Now, migratory species are more
highly specialised than resident species as regards food, and are
affected more by variations of temperature, so that they can live for
only a part of the year in the countries which they visit for the
purpose of procreation. Hence the organic changes, which set the whole
process in motion, must be coincident in time with the growth of
appropriate conditions in the environment; for if it were not so, if the
internal organic changes were to develop prematurely, the bird would
undertake its journey only to find an insufficiency of food upon its
arrival, and this would scarcely contribute towards survival. Definite
limitations have therefore been imposed upon the period of organic
change. But in the case of many resident species the conditions are
somewhat different, for they remain in the same locality throughout the
year, and a gradual unfolding of the reproductive process cannot
therefore have a similarly harmful effect. Thus it comes about that the
behaviour of the migrant, when it arrives at the breeding ground and
first falls under observation, represents a stage in the process which,
in the case of the resident, is only reached by slow degrees; and by
clos