like those of birds. And as the structure of the limbs of several of the
gigantic _Ornithoscelida_, such as _Iguandon_, leads to the conclusion
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 aviform 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 Palæozoic forms
through which that transition was actually effected.

[Illustration: FIG. 8.--PTERODACTYLUS SPECTABILIS (Von Meyer).]

We are not in a position to say that the known _Ornithoscelida_ are