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 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_.

These are the _Pterosauria_, or Pterodactyles, the remains of which are
met with throughout the series of Mesozoic rocks, from the lias to the
chalk, and some of which attain a great size, their wings having a span
of eighteen or twenty feet. These animals, in the form and proportions
of the head and neck relatively to the body, and in the fact that the
ends of the jaws were often, if not always, more or less extensively
ensheathed in horny beaks, remind us of birds. Moreover, their bones
contained air cavities, rendering them specifically lighter, as is the
case in most birds. The breast-bone was large and keeled, as in most
birds and in bats, and the shoulder girdle is strikingly similar to that
of ordinary birds. But it seems to me that the special resemblance of