less powerful than another.

So that we have two impulses operating at different seasons and guiding
the behaviour into widely divergent channels. But though the proximate
end to which the behaviour is directed is apparently different, there
are not two biological ends in view, but one--the attainment of
reproduction; and the changes that we witness are not contrary but
complementary, and their prospective value lies in the circumstance that
they contribute towards the preservation of the race.

If, then, every male is driven by inherited impulse to seek the
appropriate breeding ground each recurring season; if, having arrived
there, it is driven to seek a position of its own; if, in order to
secure isolation it is obliged to attack other males or to ward off the
attacks of intruders; if, in short, success can only be attained
providing that the inherited nature is so adjusted that the bird can
accomplish all that is here demanded--what will be the general result?
That the individual will rear its offspring in safety and that they will
inherit the peculiarities of their parents, enabling them, in their
turn, to procreate their kind; all this will certainly follow. We are
not concerned, however, at the moment, with the direct effect upon the
individual, but with the consequences that will accrue to the species as
a whole.

Now certain facts are presented to observation which enable us not only
to understand the nature of the change that is wrought in the history of
the species, but to foreshadow, with no small degree of certainty, the
extent of that change. I suppose that it has come within the experience
of most of us to observe, at one time or another, the ebb and flow of a
given species in a given district. Some favourite haunt is deserted for
a year, or for a term of years, and is then revisited; or, if it is
always occupied, the number of inhabitants fluctuates--plenty of pairs
in this season, only a few in that. Many intricate relationships, both
external and internal, contribute towards this state of affairs.
Fluctuation in a downward direction, or temporary extinction, is brought
about by changes in the physical world, by changes in the available
supply of food, by the increase of enemies, or by adverse climatic
conditions; whilst fluctuation in an upward direction, though due
indirectly to a combination of circumstances in the external world
favourable to the survival of large numbers of individuals, is direct