206 THE BIRDS OF SHERWOOD FOREST. 
have ascertained, as far as my inquiries have extended, 
that the possession of one gland is confined (with some 
exceptions, which I will mention presently) to those 
birds which are strictly land feeders; while those pro- 
vided with two seek their food on the water. 
It may, perhaps, be considered that this fact goes to 
strengthen the common opinion; but let us examine it 
a little closer. First, it is necessary for the maintenance 
of this theory that the gland or glands should contain a 
supply of oil sufficient for the constant requirements of 
the birds; and this, in those whose time is spent upon. 
the water, must be very large, for a daily application at 
least would be needful to keep the feathers as repellant 
as is requisite, to enable them to obtain their food and 
to preserve that warmth and dryness which is essential 
to their existence. But let any person ascertain by 
actual experiment the quantity of oil contained in one 
or both glands, and it will be seen at once that it is 
totally insufficient for the alleged purpose, even in those 
water birds which have the glands very large, as the 
divers and grebes. Willughby, I think, says that these 
species, and such as want tails, have the glands small ; 
but my experience is quite the contrary, and I believe 
that the tail has nothing at all to do with the 
question. 
. But there is another difficulty. Supposing the ordi- 
nary theory to be true, then all birds, and especially the 
aquatic species, would make constant application to the 
glands during the process of preening their feathers, in 
order to obtain thence the oil for dressing them. This 
I can venture to affirm is not the case. I have watched 
various birds, both wild and tame, during this operation, 
a 
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