stimulus? Through awareness of something in the environment the male responds to stimulation, and the only reply we can give is that the headquarters, or a distant song, or the proximity of another male--with all of which, as we have seen, the instinct is definitely related--are the specific factors which normally evoke response--and experience teaches us that the periods of quiescence are just those when life is at its lowest ebb and these stimulating factors less in evidence. Bearing this in mind, bearing in mind the fact that when a male joins the flock or crosses the boundary its instinct ceases to respond, bearing in mind, that is to say, that there is evidence of relationship between these specific factors and the song only when the territory is actually occupied, the conclusion seems inevitable that we have here the determining condition which renders the instinct susceptible to appropriate stimulation. There remains the female. I place her last in order of importance, not because I regard her influence as of small consequence, but because the evidence is of a varied and complex kind, so much so that it is difficult to ascertain by observation just how far she is a situational item. It will be remembered that the only direct evidence we had of such influence was a deterioration or, in some instances, a complete cessation of vocal manifestation. Clearly then we are confronted with a relationship of a different kind from that which we have been discussing; for not only is anything in the nature of stimulation absent, but, and this is a remarkable fact, the other items in the environment which formerly evoked response no longer do so in quite the same way. Is there any awareness on the part of the male of the relation between his voice and the mate that is to be, or is it merely that as the sexual situation increases in complexity some inhibiting influence comes into play? These are questions which lead up to difficult problems. But it is no part of my task to discuss the psychological aspect of the behaviour; my purpose is merely to show that the situation on the arrival of a female undergoes marked modification, that the instinct of the male is then less susceptible to stimulation, and that the factors in the external environment which formerly elicited response become relatively neutral. Hence the appearance of the female on the scene marks the opening of a new stage in the life-history of the male, and, to judge by the course of events, it would seem as if the song with its network of relationships had now served its main biological purpose.