ible than our knowledge of
recent forms might have led us to believe; and that many structural
permutations and combinations, of which the present world gives us no
indication, may nevertheless have existed.

But it by no means follows, because the _Palaeotherium_ has much in
common with the horse, on the one hand, and with the rhinoceros on the
other, that it is the intermediate form through which rhinoceroses have
passed to become horses, or _vice versa;_ on the contrary, any such
supposition would certainly be erroneous. Nor do I think it likely that
the transition from the reptile to the bird has been effected by such
a form as _Archaeopteryx._ And it is convenient to distinguish these
intermediate forms between two groups, which do not represent the actual
passage from the one group to the other, as _intercalary_ types, from
those _linear_ types which, more or less approximately, indicate the
nature of the steps by which the transition from one group to the other
was effected.

I conceive that such linear forms, constituting a series of natural
gradations between the reptile and the bird, and enabling us to
understand the manner in which the reptilian has been metamorphosed
into the bird type, are really to be found among a group of ancient and
extinct terrestrial reptiles known as the _Ornithoscelida._ The remains
of these animals occur throughout the series of mesozoic formations,
from the Trias to the Chalk, and there are indications of their
existence even in the later Palaeozoic strata.

Most of these reptiles, at present known, are of great size, some having
attained a length of forty feet or perhaps more. The majority resembled
lizards and crocodiles in their general form, and many of them were,
like crocodiles, protected by an armour of heavy bony plates. But, in
others, the hind limbs elongate and the fore limbs shorten, until
their relative proportions approach those which are observed in the
short-winged, flightless, ostrich tribe among birds.

The skull is relatively light, and in some cases the jaws, though
bearing teeth, are beak-like at their extremities and appear to have
been enveloped in a horny sheath. In the part of the vertebral column
which lies between the haunch bones and is called the sacrum, a number
of vertebrae may unite together into one whole, and in this respect,
as in some details of its structure, the sacrum of these reptiles
approaches that of birds.

But it is in the structu