The doctrine of physical uniformity and that of physical progression are therefore perfectly consistent, if we regard geological time as having the same relation to pre-geological time as historical time has to it.
The accepted doctrines of palæontology are by no means in harmony with these tendencies of physical geology. It is generally believed that there is a vast contrast between the ancient and the modern organic worlds—it is incessantly assumed that we are acquainted with the beginning of life, and with the primal manifestation of each of its typical forms: nor does the fact that the discoveries of every year oblige the holders of these views to change their ground, appear sensibly to affect the tenacity of their adhesion.
Without at all denying the considerable positive differences which really exist between the ancient and the modern forms of life, and leaving the negative ones to be met by the other lines of argument, an impartial examination of the facts revealed by palæontology seems to show that these differences and contrasts have been greatly exaggerated.
Thus, of some two hundred known orders of plants, not one is exclusively fossil. Among animals, there is not a single totally extinct class; and of the orders, at the outside not more than seven per cent. are unrepresented in the existing creation.
Again, certain well marked forms of living beings have existed through enormous epochs, surviving not only the changes of physical conditions, but persisting comparatively unaltered, while other forms of life have appeared and disappeared. Some forms may be termed “persistent types” of life; and examples of them are abundant enough in both the animal and the vegetable worlds.
Among plants, for instance, ferns, club mosses, and Coniferæ, some of them apparently generically identical with those now living, are met with as far back as the carboniferous epoch; the cone of the oolitic Araucaria is hardly distinguishable from that of existing species; a species of Pinus has been discovered in the Purbecks, and a walnut (Juglans) in the cretaceous rocks.[61] All these are types of vegetable structure, abounding at the present day; and surely it is a most remarkable fact to find them persisting with so little change through such vast epochs.
Every sub-kingdom of animals yields instances of the same kind. The Globigerina of the Atlantic soundings is identical with the cretaceous species of the same genus; and the casts of lower Silurian Foraminifera, recently described by Ehrenberg, assure us of the very close resemblance between the oldest and the newest forms of many of the Protozoa.
Among the Cœlenterata, the tabulate corals of the Silurian epoch are wonderfully like the millepores of our own seas, as every one may convince himself who compares Heliolites with Heliopora.
Turning to the Mollusca, the genera Crania, Discina, Lingula, have persisted from the Silurian epoch to the present day, with so little change, that very competent malacologists are sometimes puzzled to distinguish the ancient from the modern species. Nautili have a like range, and the shell of the liassic Loligo is similar to that of the “squid” of our own seas. Among the Annulosa, the carboniferous insects are in several cases referable to existing genera, as are the Arachnida, the highest group of which, the scorpions, is represented in the coal by a genus differing from its living congeners only in the disposition of its eyes.
The vertebrate sub-kingdom furnishes many examples of the same kind. The Ganoidei and Elasmobranchii are known to have persisted from at least the middle of the Palæozoic epoch to our own times, without exhibiting a greater amount of deviation from the typical characters of these orders, than may be found within their limits at the present day.