l60 ON THE CLASSIFICATION OP BIRDS. 
branaceous web, which extends to their extremities ; by 
this means it is transformed into a paddle or oar, 
and it is found that all birds having a foot thus con- 
structed, are better swimmers than those of any other 
order. Superiority in diving, however, seems to require 
a structure of foot more similar to that of the coot ; for 
in this respect no genus is so highly gifted as the 
grebes {fig. 87-) 
(138.) The webbed form of foot is the most prevalent 
in the natatorial order ; but it has diree or four modifi- 
cations which deserve notice. When this structure is ac- 
companied with a more than usual length of tarsus, and 
the thigh is naked just above the knee, we may feel 
assured that the power of walking is superadded to 
that of swimming. Every one knows how awkwardly 
a duck walks, or rather waddles, in comparison to the 
goose : this difference entirely arises from the length of 
their respective tarsi ; tliat of the former is very short ; 
tliat of the latter much longer. Hence the flamingo, 
which has the longest legs in the Natatorial order, is so 
good a walker that it only swims occasionally. It is 
by this formation that the gulls so materially differ from 
their congeners the terns, whose very short and weak 
legs are incapable of traversing the shore in search of 
those marine animals which the gulls walk after, and 
collect at each ebb of the tide. We have seen that the 
grebes, which are the most expert divers, have their 
hind toe broad and lobed. As an additional proof tliat 
such a form, although not essential to swimming, is yet, 
in some inexplicable way or other, very important to 
great powers of diving, we need only look to the oceanic 
ducks of the sub-family FuUgulina, which are only dis- 
tinguished in external structure from their brethren of 
the rivers (Anathi^) by having their hind toe broad 
and lobed, which the fresh water ducks have not. It is 
well known that, while the former ihve with great ex- 
pertness, die latter do so but seldom. 
( 139 .) In the penguins and the cormorants the 
hinder toe differs from that of all the other natatorial 
