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 organization, than the existing Heliopora.  As for the Aporose Corals, in what respect is the Silurian Paloeocydus less highly organized 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 specialized, than the palaeozoic Spirifer; or the existing Rhynchonellae, Craniae, Discinae, Lingulae, than the Silurian species of the same genera?  In what sense can Loligo or Spirula be said to be more specialized, or less embryonic, than Belemnites; or the modern species of Lamellibranch and Gasteropod genera, than the Silurian species of the same genera?

The ANNULOSA.—­The Carboniferous Insecta and Arachnida are neither less specialized, nor more embryonic, than those that now live, nor are the Liassic Cirripedia and Macrura; while several of the Brachyura, which appear in the Chalk, belong to existing genera; and none exhibit either an intermediate, or an embryonic, character.