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 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 specialised, nor more embryonic, than these 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.
The VERTEBRATA.—Among fishes I have referred to the Coelacanthini (comprising the genera Coelacanthus, Holophagus, Undina, and Macropoma) as affording an example of a persistent type; and it is most remarkable to note the smallness of the differences between any of these fishes (affecting at most the proportions of the body and fins, and the character and sculpture of the scales), notwithstanding their enormous range in time. In all the essentials of its very peculiar structure, the Macropoma of the Chalk is identical with the Coelacanthus of the Coal. Look at the genus Lepidotus, again, persisting without a modification of importance from the Liassic to the Eocene formations inclusively.
Or among the Teleostei—in what respect is the Beryx of the Chalk more embryonic, or less differentiated, than Beryx lineatus of King George's Sound?
Or to turn to the higher Vertebrata—in what sense are the Liassic
Chelonia inferior to those which now exist? How are the Cretaceous
Ichthyosauria, Plesiosauria, or Pterosauria less embryonic, or more
differentiated, species than those of the Lias?
Or lastly, in what circumstance is the Phascolotherium more embryonic, or of a more generalised type, than the modern Opossum; or a Lophiodon, or a Paloeotherium, than a modern Tapirus or Hyrax?
These examples might be almost indefinitely multiplied, but surely they are sufficient to prove that the only safe and unquestionable testimony we can procure—positive evidence—fails to demonstrate any sort of progressive modification towards a less embryonic, or less generalised, type in a great many groups of animals of long-continued geological existence. In these groups there is abundant evidence of variation—none of what is ordinarily understood as progression; and, if the known geological record is to be regarded as even any considerable fragment of the whole, it is inconceivable that any theory of a necessarily progressive development can stand, for the numerous orders and families cited afford no trace of such a process.
But it is a most remarkable fact, that, while the groups which have been mentioned, and many besides, exhibit no sign of progressive modification, there are others, co-existing with them, under the same conditions, in which more or less distinct indications of such a process seems to be traceable. Among such indications I may remind you of the predominance of Holostome Gasteropoda in the older rocks as compared with that of Siphonostone Gasteropoda in the later. A case less open to the objection of negative evidence, however, is that afforded by the Tetrabranchiate Cephalopoda, the forms of the shells and of the septal sutures exhibiting a certain increase of complexity in the newer genera. Here, however, one is met at once with the occurrence of Orthoceras and Baculites at the two ends of the series, and of the fact that one of the simplest genera, Nautilus, is that which now exists.