REED WARBLER
moreover, they would have recovered from the fatigue of their long journey, and would thus be in a better position to struggle when challenged by a later arrival; Nature's process is therefore a simple one, requiring no recognition on the part of the males of territories as such; their whole behaviour can be accounted for otherwise—and, what is more important still, without our having recourse to complex principles of explanation.
A male, however, does not always have to fight for its territory. It happens, and not infrequently, that its rights to ownership are recognised, and only when these rights are disputed, either by a later arrival, or by another male arriving simultaneously, does a struggle ensue. Thus we have the spectacle, as in the case of the Blackcap, of two or three males following one another for a short time after their arrival, finally separating amicably and taking possession of adjacent territories.
A truer definition of the order of migration would be this: Some males arrive before others, and some females arrive before others, but males arrive before females. The same factor which causes males to arrive before females is possibly the cause of the discrepancy in the time of arrival of the individuals of both sexes. We can of course say of the females, as we can of the males, that the earlier arrivals are probably the more vigorous; but this does not explain, any more than in the case of the males, why one individual hurries off before another. Moreover, if we accept such an explanation, we are at once faced with a difficulty, and a very considerable one, namely, that the stronger females and the weaker males will travel in company. And if, for the moment, we assume that territory has no control over reproduction, what incentive would there be for the stronger females to seek the stronger males scattered throughout the different districts, and consequently what could possibly prevent their breeding with the weaker males, thereby frustrating the very result for which we all believe Nature is for ever striving, the mainten-
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