Scientific Memoirs II
 The successive modifications which the views of physical geologists have undergone since the infancy of their science, with regard to the amount and the nature of the changes which the crust of the globe has suffered, have all tended in one direction, viz., towards the establishment of the belief, that throughout that vast series of ages which was occupied by the deposition of the stratified rocks, and which may be called "geological time" (to distinguish it from the "historical time" which followed, and the "pre-geological time," which preceded it), the intensity and the character of the physical forces which have been in operation, have varied within but narrow limits; so that, even in Silurian or Cambrian times, the aspect of physical nature must have been much what it is now.
This uniformitarian view of telluric conditions, so far as geological time is concerned, is, however, perfectly consistent with the notion of a totally different state of things in antecedent epochs, and the strongest advocate of such "physical uniformity" during the time of which we have a record might, with perfect consistency, hold the so-called "nebular hypothesis," or any other view involving the conception of a long series of states very different from that which we now know, and whose succession occupied pre-geological time.
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 worldsit 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. Such 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 Arancaria 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.1 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 subkingdom 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 Coelenterata, 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 Lolgo 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 subkingdom 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.
Among the Reptilia, the highest group, that of the Crocodilia, was represented at the beginning of the Mesozoic epoch, if not earlier, by species identical in the essential character of their organization with those now living, and presenting differences only in such points as the form of the articular faces of their vertebræ, in the extent to which the nasal passages are separated from the mouth by bone, and in the proportions of the limbs. Even such imperfect knowledge as we possess of the ancient mammalian fauna leads to the belief that certain of its types, such as that of the Marsupialia, have persisted with no greater change through as vast a lapse of time.
It is difficult to comprehend the meaning of such facts as these, if we suppose that each species of animal and plant, or each great type of organization, was formed and placed upon the surface of the globe at long intervals by a distinct act of creative power; and it is well to recollect that such an assumption is as unsupported by tradition or revelation as it is opposed to the general analogy of Nature.
If, on the other hand, we view "Persistent Types," in relation to that hypothesis which supposes the species of living beings living at any time to be the result of the gradual modification of pre-existing speciesa hypothesis which, though unproven and sadly damaged by some of its supporters, is yet the only one to which physiology lend any countenancetheir existence would seem to show, that the  amount of modification which living beings have undergone during geological time is but very small in relation to the whole series of changes which they have suffered. In fact, palæontology and physical geology are in perfect harmony, and coincide in indicating that all we know of the conditions in our world during geological time, is but the last term of a vast and, so far as our present knowledge reaches, unrecorded progression.
1 I state these facts onthe authority of my friend Dr. Hooker.
1 I state these facts onthe authority of my friend Dr. Hooker.
[Another paper of 1859, On the Anatomy and Development of Pyrosoma (Scientific Memoirs II, 318-387) offers 62 drawings depicting stages of this marine invertebrate's growth and incidentally depicting T. H. Huxley's talent as biological illustrator. For example]