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 _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_.