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.