under similar circumstances, a deterioration so marked that we learn by experience to regard it as a certain indication of the arrival of a mate.
Thus it becomes clear that there are certain specific factors in the external environment with which the instinct can be definitely related, and in the order of their importance they are (1) the territory as a whole; (2) the headquarters; (3) an intruding male; (4) the female. To what extent are these relationships interrelated? Are they all mutually dependent upon one another, or is there one which conditions the remainder?
In the first place it is evident that if a male were not to establish a territory, no opportunity would be afforded for making use of any special post or for acquiring a habit in relation to it, and so without further consideration we may say that the connection between the song and the headquarters, whatever it may be, is primarily dependent upon the establishment of a territory.
Next, we have the fact that the distant voice, or still more so the presence, of another male has an exciting influence and evokes a corresponding reply. Here we have a direct relationship, and one which at first sight appears to be exclusive of cross-correlation. But is it really so J does no circumstance arise under which even the proximity of a rival fails to evoke response? The reply is not doubtful. Such a circumstance does arise—when a male for one reason or another
