372 THE ZOOLOGIST. 



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 catar- 

 rhactes), 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 compara- 

 tively 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 repre- 

 sented 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 



