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other bird circled at a still greater height, and never once joined in the attack. The height, I may say, from which the birds swoop is not, as a rule, very considerable. The above does not apply equally to the Arctic skua—at least in my own experience—for though often the two birds would attack, yet in the greater number of cases only one of them did so. Now the Arctic skua, as I have mentioned elsewhere, is one of those birds which employs strategy (begging here the question for the sake of brevity) as well as force to defend its young, and it occurred to me that here might be a case of co-operation, the male bird most probably attacking, and the female employing the ruse. I satisfied myself, however, that the same bird sometimes does both one and the other. How often this is so, and whether there is a tendency on the part of either sex to resort by preference to one or the other method, it might be difficult to find out. Yet I cannot help thinking that this is the case, and that a process of differentiation is in course of taking place. The facts are—or appeared to me to be—these. In the case of the great skua, both sexes—almost, but not quite, always—attack, and there is no ruse. In that of the Arctic skua both sexes sometimes attack, but far more frequently (that, at least, was my own experience) one alone does so, and here a ruse is employed. In the former case we just see occasionally, as an exception, the raw material (the non-attacking of the one bird) that might conceivably be utilised by nature for the elaboration of another form of defence. In the latter we may see this other form being elaborated.