The Articulates were largely represented in this epoch. There were already in the vegetation a number of Gymnosperms, affording more favorable nourishment for Insects than the forests of earlier times; and we accordingly find that class in larger numbers than ever before, though still meagre in comparison with its present representation. Crustacea were numerous,—those of the Shrimp and Lobster kinds prevailing, though in some of the Lobsters we have the first advance towards the highest class of Crustacea in the expansion of the transverse diameter now so characteristic of the Crabs. Among Mollusks we have a host of gigantic Ammonites; and the naked Cephalopods, which were in later times to become the prominent representatives of that class, already begin to make their appearance. Among Radiates, some of the higher kinds of Echinoderms, the Ophiurans and Echinolds, take the place of the Crinoids, and the Acalephian Corals give way to the Astræan and Meandrina-like types, resembling the Reef-Builders of the present time.
I have spoken especially of the inhabitants of the Jurassic sea lying between England and France, because it was there that were first found the remains of some of the most remarkable and largest Jurassic animals. But wherever these deposits have been investigated, the remains contained in them reveal the same organic character, though, of course, we find the land Reptiles only where there happen to have been marshes, the aquatic Saurians wherever large estuaries or bays gave them an opportunity of coming in near shore, so that their bones were preserved in the accumulations of mud or clay constantly collecting in such localities,—the Crustacea, Shells, or Sea-Urchins on the old sea-beaches, the Corals in the neighborhood of coral reefs, and so on. In short, the distribution of animals then as now was in accordance with their nature and habits, and we shall seek vainly for them in the localities where they did not belong.
But when I say that the character of the Jurassic animals is the same, I mean, that, wherever a Jurassic sea-shore occurs, be it in France, Germany, England, or elsewhere throughout the world, the Shells, Crustacea, or other animals found upon it have a special character, and are not to be confounded by any one thoroughly acquainted with these fossils with the Shells or Crustacea of any preceding or subsequent time,—that, where a Jurassic marsh exists, the land Reptiles inhabiting it are Jurassic, and neither Triassic nor Cretaceous,—that a Jurassic coral reef is built of Corals belonging as distinctly to the Jurassic creation as the Corals on the Florida reefs belong to the present creation,—that, where some Jurassic bay or inlet is disclosed to us with the Fishes anciently inhabiting it, they are as characteristic of their time as are the Fishes of Massachusetts Bay now.
And not only so, but, while this unity of creation prevails throughout the entire epoch as a whole, there is the same variety of geographical distribution, the same circumscription of faunæ within distinct zoölogical provinces, as at the present time. The Fishes of Massachusetts Bay are not the same as those of Chesapeake Bay, nor those of Chesapeake Bay the same as those of Pamlico Sound, nor those of Pamlico Sound the same as those of the Florida coast. This division of the surface of the earth into given areas within which certain combinations of animals and plants are confined is not peculiar to the present creation, but has prevailed in all times, though with ever-increasing diversity, as the surface of the earth itself assumed a greater variety of climatic conditions. D'Orbigny and others were mistaken in assuming that faunal differences have been introduced only in the last geological epochs. Besides these adjoining zoölogical faunæ, each epoch is divided, as we have seen, into a number of periods, occupying successive levels one above another, and differing specifically from each other in time as zoölogical provinces differ from each other in space. In short, every epoch is to be looked upon from two points of view: as a unit, complete in itself, having one character throughout, and as a stage in the progressive history of the world, forming part of an organic whole.
As the Jurassic epoch was ushered in by the upheaval of the Jura, so its close was marked by the upheaval of that system of mountains called the Côte d'Or. With this latter upheaval began the Cretaceous epoch, which we will examine with special reference to its subdivision into periods, since the periods in this epoch have been clearly distinguished, and investigated with especial care. I have alluded in the preceding article to the immediate contact of the Jurassic and Cretaceous epochs in Switzerland, affording peculiar facilities for the direct comparison of their organic remains. But the Cretaceous deposits are well known, not only in this inland sea of ancient Switzerland, but in a number of European basins, in France, in the Pyrenees, on the Mediterranean shores, and also in Syria, Egypt, India, and Southern Africa, as well as on our own continent. In all these localities, the Cretaceous remains, like those of the Jurassic epoch, have one organic character, distinct and unique. This fact is especially significant, because the contact of their respective deposits is in many localities so immediate and continuous that it affords an admirable test for the development-theory. If this is the true mode of origin of animals, those of the later Jurassic beds must be the progenitors of those of the earlier Cretaceous deposits. Let us see now how far this agrees with our knowledge of the physiological laws of development.
Take first the class of Fishes. We have seen that in the Jurassic periods there were none of our common Fishes, none corresponding to our Herring, Pickerel, Mackerel, and the like,—no Fishes, in short, with thin membranous scales, but that the class was represented exclusively by those with hard, flint-like scales. In the Cretaceous epoch, however, we come suddenly upon a horde of Fishes corresponding to our smaller common Fishes of the Pickerel and Herring tribes, but principally of the kinds found now in tropical waters; there are none like our Cods, Haddocks, etc., such as are found at present in the colder seas. The Fishes of the Jurassic epoch corresponding to our Sharks and Skates and Gar-Pikes still exist, but in much smaller proportion, while these more modern kinds are very numerous. Indeed, a classification of the Cretaceous Fishes would correspond very nearly to one founded on those now living. Shall we, then, suppose that the large reptilian Fishes of the Jurassic time began suddenly to lay numerous broods of these smaller, more modern, scaly Fishes? And shall we account for the diminution of the previous forms by supposing that in order to give a fair chance to the new kinds they brought them forth in large numbers, while they reproduced their own kind less abundantly? According to very careful estimates, if we accept this view, the progeny of the Jurassic Fishes must have borne a proportion of about ninety per cent, of entirely new types to some ten per cent, of those resembling the parents. One would like a fact or two on which to rest so very extraordinary a reversal of all known physiological laws of reproduction, but, unhappily, there is not one.
Still more unaccountable, upon any theory of development according to ordinary laws of reproduction, are those unique, isolated types limited to a single epoch, or sometimes even to a single period. There are some very remarkable instances of this in the Cretaceous deposits. To make my statement clearer, I will say a word of the sequence of these deposits and their division into periods.
These Cretaceous beds were at first divided only into three sets, called the Neocomian, or lower deposits, the Green-Sands, or middle deposits, and the Chalk, or upper deposits. The Neocomian, the lower division, was afterwards subdivided into three sets of beds, called the Lower, Middle, and Upper Neocomian by some geologists, the Valengian, Neocomian, and Urgonian by others. These three periods are not only traced in immediate succession, one above another, in the transverse cut before described, across the mountain of Chaumont, near Neufchatel, but they are also traced almost on one level along the plain at the foot of the Jura. It is evident that by some disturbance of the surface the eastern end of the range was raised slightly, lifting the lower or Valengian deposits out of the water, so that they remain uncovered, and the next set of deposits, the Neocomian, is accumulated along their base, while these in their turn are slightly raised, and the Urgonian beds are accumulated against them a little lower down. They follow each other from east to west in a narrower area, just as the Azoic, Silurian, and Devonian deposits follow each other from north to south in the northern part of the United States. The Cretaceous deposits have been intimately studied in various localities by different geologists, and are now subdivided into at least ten, or it may be fifteen or sixteen distinct periods, as they stand at present. This is, however, but the beginning of the work; and the recent investigations of the French geologist, Coquand, indicate that several of these periods at least are susceptible of further subdivision. I present here a table enumerating the periods of the Cretaceous epoch best known at present, in their sequence, because I want to show how sharply and in how arbitrary a manner, if I may so express it, new forms are introduced. The names are simply derived from the localities, or from some circumstances connected with the locality where each period has been studied.