What
their number may have been is uncertain; but several, if not all, of
them were terminated by strong curved claws, not like such as are
sometimes found in birds, but such as reptiles possess; so that, in the
_Archaeopteryx_, we have an animal which, to a certain extent, occupies a
midway place between a bird and a reptile. It is a bird so far as its
foot and sundry other parts of its skeleton are concerned; it is
essentially and thoroughly a bird by its feathers; but it is much more
properly a reptile in the fact that the region which represents the hand
has separate bones, with claws resembling those which terminate the
fore-limb of a reptile. Moreover, it had a long reptile-like tail with a
fringe of feathers on each side; while, in all true birds hitherto
known, the tail is relatively short, and the vertebrae which constitute
its skeleton are generally peculiarly modified.
Like the _Anoplotherium_ and the _Palaeotherium_, therefore,
_Archaeopteryx_ tends to fill up the interval between groups which, in
the existing world, are widely separated, and to destroy the value of
the definitions of zoological groups based upon our knowledge of
existing forms. And such cases as these constitute evidence in favour of
evolution, in so far as they prove that, in former periods of the
world's history, there were animals which overstepped the bounds of
existing groups, and tended to merge them into larger assemblages.
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