Again, what can be more remarkable than the singular constancy of
structure preserved throughout a vast period of time by the family of
the Pycnodonts and by that of the true Coelacanths; the former
persisting, with but insignificant modifications, from the
Carboniferous to the Tertiary rocks, inclusive; the latter existing,
with still less change, from the Carboniferous rocks to the Chalk,
inclusive?

Among Reptiles, the highest living group, that of the Crocodilia, is
represented, at the early part of the Mesozoic epoch, by species
identical in the essential characters of their organization with those
now living, and differing from the latter only in such matters as the
form of the articular facets of the vertebral centra, in the extent to
which the nasal passages are separated from the cavity of the mouth by
bone, and in the proportions of the limbs.

And even as regards the Mammalia, the scanty remains of Triassic and
Oolitic species afford no foundation for the supposition that the
organization of the oldest forms differed nearly so much from some of
those which now live as these differ from one another.

It is needless to multiply these instances; enough has been said to
justify the statement that, in view of the immense diversity of known
animal and vegetable forms, and the enormous lapse of time indicated by
the accumulation of fossiliferous strata, the only circumstance to be
wondered at is, not that the changes of life, as exhibited by positive
evidence, have been so great, but that they have been so small.

Be they great or small, however, it is desirable to attempt to estimate
them. Let us, therefore, take each great division of the animal world