Questions of this nature might be settled in the future on facts observed now, as easily as a reference to an iron ring where boats were once moored settles the question as to whether the coast has risen or the sea encroached. The coast and the sea, however, remain. Birds, slaughtered by millions each year, must cease almost as a class before any great period has gone by. Of what use then the ring, the record when what it speaks of is no more?
Another interesting point in the Arctic skua (which it shares with at least one other species of the genus) is its dimorphism—or rather, to describe it more properly, its polymorphism. To me it seems to offer a case of a species in course of variation from one form into another. In the two extreme forms the plumage is, respectively, either entirely sombre both above and below, or the whole throat, breast and under surface, with a ring round the neck, and more or less of the sides of the head, is of a fine cream colour. Between these extremes there are various gradations, the cream being sometimes on the breast only, whilst the throat is of a lighter or deeper grey, more or less mottled with the still darker shade, or the lighter colour is hardly or not at all discernible on these parts, whilst lower down it becomes less and less salient till it is merely a not so dusky duskiness. The cream-coloured birds, though numerous, are in the minority, and both this and their being much handsomer suggests that the process of change is in this direction, whilst the intermediate tintings may represent the steps in this process. To what form of selection (if to any) are we to attribute the change? As the cream colouring makes the bird more con-
