The footprints now
under consideration form a double series of _single_ prints, and
therefore, beyond all question, are the tracks of a _biped_--that
is, of an animal which walked upon two legs. No living animals,
save Man and the Birds, walk habitually on two legs; and there
is, therefore, a _prima facie_ presumption that the authors of
these prints were Birds. Moreover, each impression consists of
the marks of three toes turned forwards (fig. 155), and therefore
are precisely such as might be produced by Wading or Cursorial
Birds. Further, the impressions of the toes show exactly the
same numerical progression in the number of the joints as is
observable in living Birds--that is to say, the innermost of the
three toes consists of three joints, the middle one of four, and
the outer one of five joints. Taking this evidence collectively,
it would have seemed, until lately, quite certain that these
tracks could only have been formed by Birds. It has, however,
been shown that the Deinosaurian Reptiles possess, in some cases
at any rate, some singularly bird-like characters, amongst which
is the fact that the animal possessed the power of walking,
temporarily at least, on its hind-legs, which were much longer and
stronger than the fore-limbs, and which were sometimes furnished
with no more than three toes. As the bones and teeth of Deinosaurs
have been found in the Triassic deposits of North America, it
may be regarded as certain that _some_ of the bipedal tracks
originally ascribed to Birds must have really been produced by
these Reptiles.
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