might
not that be as injurious to the young as prolonged exposure? Manifestly
the impulse to brood could only have developed strength in so far as it
fitted in with all the other factors that make for survival; and the
principal factor in the external environment seems to be the territory.
How could the young have been freed from the risk of exposure if the
impulse to brood had not been so strongly implanted in the parent? How
could the impulse to brood have been free to develop if a supply of food
had not been first insured? How could the supply of food have been
insured if numbers of the same species had been allowed to breed in
close proximity?

From the foregoing facts it is clear that the young of many species are
at birth susceptible to cold and unable to withstand prolonged exposure.
The parents must therefore be in a position to obtain food rapidly, and
consequently it is important that there should be an ample supply in the
vicinity of the nest. This end the territory certainly serves to
promote; it roughly insures that the bird population of a given area is
in proportion to the available means of subsistence, and it thus reduces
the risk of prolonged exposure to which the young are always liable.

This leads on to a consideration of those cases in which the question of
securing food is subordinate to the question of securing a station
suitable for reproduction.

I take the Guillemot as an example. In principle its behaviour is
similar to that of the Bunting; the male repairs to a definite place,
isolates itself, and becomes pugnacious. But the Guillemot is generally
surrounded by other Guillemots, and the birds are often so densely
packed along the ledges that there is scarcely standing room, so it
seems, for all of them. Nevertheless the isolation of the individual is,
in a sense, just as complete as that of the individual Bunting, for each
one is just as vigilant in resisting intrusion upon its few square feet
as the Bunting is in guarding its many square yards, so that the
evidence seems to show that that part of the inherited nature which is
the basis of the territory is much the same in both species. What we
have then to consider is, What is the biological value to the Guillemot
of an inherited nature which, for the Bunting, has utility in relation
to the supply of food for the young? Up to a point, the act of securing
a territory has like value for each respective species, whether the area
occupied