recedes from the base of
the cliff at low tide. Many miles of rock-bound coast are thus useless
for the purpose of reproduction.

Now when we bear in mind how large an expanse of coast is formed of
blown sand or of rocks of low altitude, and how many miles of cliff fail
to supply the three essential conditions that we have been considering,
we can see that suitable breeding stations must be limited both in
number and extent. From a wide expanse of ocean hosts of individuals are
therefore obliged to converge at certain definite points; and hence,
each recurring season, there must arise a competition for positions at
the station, just as there is competition between individual Buntings
for positions in the marsh. And the ability to obtain a position upon a
suitable ledge involves, in the first place, an impulse to search for
it; in the second place, an impulse to dwell in it; and in the third
place, an impulse to resist intrusion upon it. It would be useless for
an individual to be pugnacious if it had no fixed abode; equally useless
for it to establish itself on a particular ledge if it had no power to
defend it--all of which implies an inherited nature similar to that of
the Bunting. But the proximate end to which the competition is directed
is not alike in the case of both species. In the case of the Guillemot
it has reference solely to the piece of rock whereon the egg is laid; in
the case of the Bunting to a piece of ground capable of furnishing an
adequate supply of food for the young; and the reason for the difference
is this, that there is always an abundance of food in the water beneath
the cliff, but breeding stations are scarce, whereas there is always an
abundance of situations in the marsh in which the Bunting can place its
nest, but the supply of food varies and at times can only be obtained
with difficulty.

If then the Guillemot were to behave after the manner of the Bunting and
assign to itself a portion of the face of the cliff, or if it were only
to occupy a few ledges, or an even lesser area--a single ledge--what
would be the result? That it would attain to reproduction is beyond
question; that the egg would be safely deposited there can be no manner
of doubt; neither is there any reason to suppose that the offspring
would not be successfully reared. But, indirectly, its behaviour would
affect the Guillemot race. For if it be true, as the crowded ledges
certainly seem to show, that there is a dearth