BRITISH WARBLERS
the individuals in some parts of the globe, we can see that it is more than probable that many districts, formerly inhabited, would not be inhabited to-day.
In applying this law of battle to the life-history of these smaller species, I am only extending a principle which is already recognised as a factor in the evolution of animal life. But in positing territory as the primary cause of the struggles I am departing from the prevailing opinion. And inasmuch, therefore, as this is the point which is the more likely to be disputed, it will be as well if we review once more, but briefly, the evidence upon which it is based, and in so doing we shall see how impossible it is to reconcile the facts with the only other possible explanation, namely, that the females, are the direct cause of the struggles. In the first place, then, we have this fact established, that in the majority of instances each individual male of the migratory species when it finally reaches its destination, and each individual male of the resident species when the sexual instinct commences to develop, and in some cases throughout the year, remains in a certain restricted area encompassed by well-defined boundaries which it seems unwilling to cross. Next we have the spectacle of these same males engaging in fierce conflicts amongst themselves, and in the case of the migratory species even before any females have arrived; and this one fact, if placed beyond dispute, would alone be sufficient to show that the females are not the direct incentive to the struggles. But as the plea of insufficient observation may here be raised— and no one can deny that a female skulking in the undergrowth might easily escape detection—we will pass on to the behaviour of the males after the females have unquestionably arrived, and we find them still continuing to struggle after pairing has taken place, after the nest is built, and even after incubation has commenced, though with less and less determination as the season advances; and, moreover, we can observe that these quarrels follow after, and not infrequently seem to be con-
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