ly 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 in
succession, and, whenever an order or a family can be shown to have had
a prolonged existence, let us endeavour to ascertain how far the later
members of the group differ from the earlier ones. If these later
members, in all or in many cases, exhibit a certain amount of
modification, the fact is so far, evidence in favour of a general law of
change; and, in a rough way, the rapidity of that change will be
measured by the demonstrable amount of modification. On the other hand,
it must be recollected that the absence of any modification, while it
may leave the doctrine of the existence of a law of change without
positive support, cannot possibly disprove all forms of that doctrine,
though it may afford a sufficient refutation of many of them.

The PROTOZOA.--The Protozoa are represented throughout the
whole range of geological series, from the Lower Silurian formation to
the present day. The most ancient forms recently made known by Ehrenberg
are exceedingly like those which now exist: no one has ever pretended
that the difference between any ancient and any modern Foraminifera is
of more than generic value; nor are the oldest Foraminifera either
simpler, more embryonic, or less differentiated, than the existing
forms.

The CoeLENTERATA.--The Tabu