hat this male has
paired, behaves similarly; she, too, flies to the surrounding country
and in time returns with equal certainty. Sometimes male and female
accompany one another--that is, they leave simultaneously and likewise
return; at other times, though they depart together, the male returns
alone; or the male may disappear in one direction whilst the female does
so in another--and, on the whole, there is a sameness in the direction
of flight taken by the same pairs on different occasions. An interval of
nearly two months may thus elapse between mating and nest-building,
during which the sexes are not only often apart but often separated by a
considerable distance.

What does this species gain by the individuals belonging to it mating so
early in the season? If the appropriate condition which leads the
females to seek males were to arise in each individual at a late date,
the first stage in the process--mating--would not be completed before
the second--the discharge of the sexual function--were due to begin.
Thus, instead of having ample time, the females would have but a short
period in which to discover males; and this in some cases might lead to
delay, in others to failure, and in others again to needlessly severe
competition, entailing physical exhaustion at a critical moment in
their lives. Hence those females in which the appropriate organic
condition developed early in the season would not only be more likely to
find males, but would be in a position to rear more broods than those in
which it developed late; and they would have a better chance of leaving
offspring, which, in their turn, would reproduce the peculiarities of
their parents. Moreover, within certain limitations, the more these
successful females varied in the date of their development, the less
severe would be the competition, and the more uniformly successful would
the mating of all the individuals in a given district tend to become.
But all of this renders an interval of sexual inactivity unavoidable; an
interval which must constitute a danger unless there were something in
the external environment to prevent the male and female from drifting
apart. Inasmuch, then, as the occupation of a territory serves to remove
all possibility of permanent separation, I suggest that its evolution
has afforded the condition under which this beneficial procedure has
developed--free to mate when they will, free to seek food where they
will, free to pursue their