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 any 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 organization, than the existing
'Heliopora'. As for the Aporose Corals, in what respect is the Silurian
'Paleocyclus' less highly organized or more embryonic than the modern
'Fungia', or the Liassic Aporosa than the existing members of the same
families?