It is not historical proof of
the occurrence of the evolution of birds from reptiles, for we have no
safe ground for assuming that true birds had not made their appearance
at the commencement of the Mesozoic epoch. It is, in fact, quite
possible that all these more or less avi-form reptiles of the Mesozoic
epoch are not terms in the series of progression from birds to reptiles
at all but simply the more or less modified descendants of Palaeozoic
forms through which that transition was actually effected.
We are not in a position to say that the known _Ornithoscelida_ are
intermediate in the order of their appearance on the earth between
reptiles and birds. All that can be said is that, if independent
evidence of the actual occurrence of evolution is producible, then these
intercalary forms remove every difficulty in the way of understanding
what the actual steps of the process, in the case of birds, may have
been.
That intercalary forms should have existed in ancient times is a
necessary consequence of the truth of the hypothesis of evolution; and,
hence, the evidence I have laid before you in proof of the existence of
such forms, is, so far as it goes, in favour of that hypothesis.
There is another series of extinct reptiles, which may be said to be
intercalary between reptiles and birds, in so far as they combine some
of the characters of both these groups; and, which, as they possessed
the power of flight, may seem, at first sight, to be nearer
representatives of the forms by which the transition from the reptile to
the bird was effected, than the _Ornithoscelida_.
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