on
that they also may have constantly, or occasionally, assumed the same
attitude, a peculiar interest attaches to the fact that, in the Wealden
strata of England, there are to be found gigantic footsteps, arranged in
order like those of the _Brontozoum_, and which there can be no
reasonable doubt were made by some of the _Ornithoscelida_, the remains
of which are found in the same rocks. And, knowing that reptiles that
walked upon their hind legs and shared many of the anatomical characters
of birds did once exist, it becomes a very important question whether
the tracks in the Trias of Massachusetts, to which I referred some time
ago, and which formerly used to be unhesitatingly ascribed to birds, may
not all have been made by Ornithoscelidan reptiles; and whether, if we
could obtain the skeletons of the animals which made these tracks, we
should not find in them the actual steps of the evolutional process by
which reptiles gave rise to birds.

The evidential value of the facts I have brought forward in this Lecture
must be neither over nor under estimated. 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;