4 ON THE CLASSIFICATION OF BIRDS. 
been well observed of the wading tribes, they command 
three elements, and can make way equally weU on land, 
in the air, or in water. They possess the greatest 
energy of respiration known to exist ; and their powers 
of vision are more highly developed than in any other 
class of vertebrated animals. Their internal structure 
is not less peculiar. Their young are produced from 
eggs, and they have a double system of circulation and 
respiration : the lungs are fixed to the ribs, undivided, 
and pierced in such a way as to permit the air not only 
to pass into the chest and body, but even to penetrate 
the interior of the bones, so that every part is im- 
pregnated with that fluid in which they are destined 
to move. 
(4.) We have already shown* that Birds, in the ver- 
tebrated circle, occupy a station between Reptiles and 
Quadrupeds. Between Birds and the former there 
seems, in the living world, to be a wide hiatus ; a gap, 
which nothing now known to exist in creation can fill 
up. But this apparent interruption is not, in fact, 
so great as may be thought ; and even if it were, it 
must not be confounded with an entanglement, or with 
a saltus, or leap, of Nature ; many links are, indeed, 
wanting, but the circles, at those parts where they may 
be supposed to touch each other, stiU preserve such a 
resemblance, and manifest such a mutual approximation, 
as to leave us in no doubt on their real and absolute 
affinity. Were we to rest satisfied with the affinity 
thought to exist between the tortoises and the penguins, 
the subject might still be left in doubt ; for analysis has 
satisfied us that this is a resemblance of analogy, and 
not of affinity. It is here, in short, that we see the 
absolute necessity of studying the forms of extinct 
animals, no less than of those now existing. In the 
extraordinary genus Ptarodactylus, there is such a sin- 
gular union of the reptile with the bird, that an ordi- 
nary observer, looking at its skeleton, would be quite at 
a loss to decide to which class it belonged. In these flying 
» On the Classification of Quadrupeds, p. 44. 
