24 THE BIRD WATCHER 
old one. Of what u@@can this thin projection, an 
inch or so beyond the serviceable fan of the tail, be 
to the bird? Seeing how well every other bird does 
without it, can we suppose it to be of any service? 
Its beauty, however—which one misses dreadfully in 
the young flying bird—is apparent to any one, and it 
goes hand in hand with an ascending scale of beauty 
in colour. All this seems to me to point strongly 
towards sexual selection as the agency by which these 
changes have been, and are being, effected ;’ since I 
am, personally, a believer in the reality of that power, 
having never heard or read anything against it, so 
convincing to my mind as what Darwin said for it, 
nor seen anything that has appeared to me to be in- 
consistent either with his facts or his arguments. 
No doubt if the varied coloration of the Arctic 
skua is really to be explained in this way, the lighter- 
coloured forms, especially the extreme one, in which 
the whole under surface is cream, ought to be on the 
increase, whilst the dark ones should ultimately die 
out or remain, perhaps, as a separate species, the inter- 
mediate tintings having disappeared. It is very 
dificult to form an idea of the relative number of 
individuals constituting any one form, because one 
unconsciously compares such form with a great many 
others instead of with each separately ; but, whereas I 
remember various repetitions of the extreme light or 
1 Tt is a strong enforcement, I think, of this view, that in another variable 
species of skua—Stercorarius pomatorhinus—the same two feathers give the bird “the 
grotesque appearance of having a disk attached to its tail.” 
