Page:The Zoologist, 4th series, vol 6 (1902).djvu/438

372 which have, in my view, been classed all together as the dark variety. Still, with all this, I confess it is a puzzle to me how a bird, the individuals of which differ so greatly and indefinitely, can have come to be considered as merely exhibiting two forms of colouring.

As far as I remember, all the nestling birds which I have seen have been merely brown, without any admixture of cream under the fluff; but I have not seen very many. When older and able to fly, but still young, all that I have seen have had a colouring of their own—for their plumage has borne a considerable resemblance to that of the Great Skua (Stercorarius catarrhactes), being mottled on the back with two shades of brown, a darker and a lighter one. I got the effect of this when I watched young birds flying or standing, and one day I caught one whose wing had been injured, and saw that it was so. This resemblance is increased by such birds wanting the lance-like feathers (or feather) in the tail. This mottled brown is the only kind of colouring which I have seen in these immature but comparatively advanced birds. Certainly, compared to the old ones, there were but few of these to be seen on my late visit. Had there been only one, however, that exhibited the ordinary light or dark form of plumage, or any sensible approach towards it, I believe I should have noticed it, as I was for seventeen days on the spot. My impression is that in the still younger birds this mottling was either absent or not so noticeable. At any rate, I have no clear recollection of it.

My own explanation of all these facts is that Stercorarius crepidatus, having been originally a plain homely-coloured bird, like the Great Skua, is being gradually modified, under the influence of sexual selection, into a most beautiful one, as represented by the extreme light, or half-cream, form. Natural selection seems here excluded, or, at any rate, extremely doubtful; and, if it be proposed that the lighter (or darker) birds have the more vigorous constitutions, I can only say that I believe it would be extremely difficult to produce any kind of evidence in favour of the suggestion. Without evidence, such a view is a mere supposition, and therefore not worth while considering. The main facts suggest choice in a certain direction. There is a gradation of colour and pattern connecting two forms—one