al 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 Tabulate Corals have existed from the Silurian
epoch to the present day, but I am not aware that the ancient
_Heliolites_ possesses a single mark of a more embryonic or less
differentiated character, or less high organisation, than the existing
_Heliopora_. As for the Aporose Corals, in what respect is the Silurian
_Paloeocyclus_ less highly organised or more embryonic than the modern
_Fungia_, or the Liassic Aporosa than the existing members of the same
families?

The _Mollusca_--In what sense is the living _Waldheimia_ less embryonic,
or more specialised, than the palaeozoic _Spirifer_; or the existing
_Rhynchonelloe, Cranioe, Discinoe, Linguloe_, than the Silurian species
of the same genera? In what sense can _Loligo_ or _Spirula_ be said to be
more specialised, or l