permutations and combinations, of which the present world gives us no
indication, may nevertheless have existed.

But it by no means follows, because the _Palæotherium_ 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 versâ_; 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 _Archæopteryx_. 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 Palæozoic 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