But apart from any direct assistance she may give in driving away intruders, does she in any way help to further the biological end of reproduction? This is a difficult question to answer, and the suggestion I have to make can only apply in those cases in which the territory is occupied throughout the breeding season. Much of the fighting between the males occurs in her presence, and it must be admitted--though it is difficult to speak with any degree of certainty--that such fighting, taken as a whole, bears the stamp of exceptional determination. Let us then grant that the excitement of a male does, under these circumstances, reach a higher level of intensity, and let us see how this will add to the security of the territory. The fact that the male has established itself and obtained a mate is not alone sufficient to accomplish the end for which the territory has been evolved. During the period between the initial discharge of the sexual function and the time when incubation draws to a close, much may happen to prejudice the future of the offspring; there is always the possibility of invasion by an individual whose development is backward or which has been unsuccessful in making good the first step, and, as we saw in the case of the Reed-Bunting, a portion of the ground won may be lost; there is always the danger of gradual encroachment by neighbouring owners; and there is even a possibility that a pair may be so persistently harassed by more virile neighbours as to forsake the locality permanently. If then a male is to attain a full measure of success it must be capable of keeping its boundaries intact up to the time when the young are able to fend for themselves, and consequently it is important that its intolerant nature should remain susceptible to stimulation throughout the greater part of the season.
Does the presence of a female serve to promote this end? Now we know very little of the influence exerted by one sex upon the other. Professor Lloyd Morgan has suggested that the male raises the emotional tone of the female, a suggestion which seems to me in accordance with the facts. There is reason to believe, however, that the converse is also true--namely that the excitement of the male reaches a higher level of intensity when a female is present. Granting then that his emotional tone is raised, how will this affect the question? So great is the difference of opinion as to the part that the emotions play in furthering the life of the individual that one hesitates to accept any particular one. But it seems to be generally admitted that emotion adds to the efficacy of behaviour, and this is the view of Professor Lloyd Morgan. "Whatever may be the exact psychological nature of the emotions, it may be regarded," he says, "as certain that they introduce into the conscious situation elements which contribute not a little to the energy of behaviour. They are important conditions to vigorous and sustained conation." Therefore, if it be true that the female raises the emotional tone of the male, the result will be an increased flow of energy into all the specific modes of behaviour connected with reproduction, amongst which those directly concerned in the securing and defence of the territory will receive their share; so that instead of a progressive weakening of just those elements in the situation which make for success, the level of their efficiency will be maintained as a result of such reinforcement. But the female becomes intolerant of her own sex when she has discovered a male ready to breed, and, later, assists her mate in resisting intrusion; and by raising her emotional tone, he may be the means of furthering more strenuous behaviour on her part. Each member of the pair would in this way contribute towards the energy of behaviour of its mate, and hence add indirectly to the security of the territory.