[THE GROWTH OF CONTINENTS.]
Before entering upon a sketch of the growth of the European Continent from the earliest times until it reached its present dimensions and outlines, I will say something of the growth of continents in general, connecting these remarks with a few words of explanation respecting some geological terms, which, although in constant use, are nevertheless not clearly defined. I will explain, at the outset, the meaning I attach to them and the sense in which I use them, that there may be no misunderstanding between me and my readers on this point. The words Age, Epoch, Period, Formation, may be found on almost every page of any modern work on geology; but if we sift the matter carefully, we shall find that there is a great uncertainty as to the significance of these terms, and that scarcely any two geologists use them in the same sense. Indeed, I shall not be held blameless in this respect myself; for, on looking over preceding articles, I find that I have, from old habit, used somewhat indiscriminately names which should have a perfectly definite and invariable meaning.
As long as zoölogical nomenclature was uncontrolled by any principle, the same vagueness and indecision prevailed here also. The words Genus, Order, Class, as well as those applied to the most comprehensive division of all in the animal kingdom, the primary branches or types, were used indiscriminately, and often allowed to include under one name animals differing essentially in their structural character. It is only since it has been found that all these groups are susceptible of limitation, according to distinct categories of structure, that our nomenclature has assumed a more precise and definite significance. Even now there is still some inconsistency among zoölogists as to the use of special terms, arising from their individual differences in appreciating, structural features; but I believe it to be, nevertheless, true, that general orders, classes, etc., are not merely larger or smaller groups of the same kind, but are really based upon distinct categories of structure. As soon as such a principle is admitted in geology, and investigators recognize certain physical and organic conditions, more or less general in their action, as characteristic of all those chapters in geological history designated as Ages, Epochs, Periods, Formations, etc., all vagueness will vanish from the scientific nomenclature of this department also, and there will be no hesitation as to the use of words for which we shall then have a positive, definite meaning.
Although the fivefold division of Werner, by which he separated the rocks into Primitive, Transition, Secondary, Alluvial, and Volcanic, proved to be based on a partial misapprehension of the nature of the earth-crust, yet it led to their subsequent division into the three great groups now known as the Primary, or Palaeozoic, as they are sometimes called, because here are found the first organic remains, the Secondary, and the Tertiary. I have said in a previous article that the general unity of character prevailing throughout these three divisions, so that, taken from the broadest point of view, each one seems a unit in time, justifies the application to them of that term, Age, by which we distinguish in human history those periods marked throughout by one prevailing tendency;—as we say the age of Egyptian or Greek or Roman civilization,—the age of stone or iron or bronze. I believe that this division of geological history into these great sections or chapters is founded upon a recognition of the general features by which they are characterized.
Passing over the time when the first stratified deposits were accumulated under a universal ocean in which neither animals nor plants existed, there was an age in the physical history of the world when the lands consisted of low islands,—when neither great depths nor lofty heights diversified the surface of the earth,—when both the animal and vegetable creation, however numerous, was inferior to the later ones, and comparatively uniform in character,—when marine Cryptogams were the highest plants, and Fishes were the highest animals. And this broad statement holds good for the whole of that time, even though it was not without its minor changes, its new forms of animal and vegetable life, its variations of level, its upheavals and subsidences; for, nevertheless, through its whole duration, it was the age of low detached lands,—it was the age of Cryptogams,—it was the age of Fishes. From its beginning to its close, no higher type in the animal kingdom, no loftier group in the vegetable world, made its appearance.
There was an age in the physical history of the world when the patches of land already raised above the water became so united as to form large islands; and though the aspect of the earth retained its insular character, yet the size of the islands, their tendency to coalesce by the addition of constantly increasing deposits, and thus to spread into wider expanses of dry land, marked the advance toward the formation of continents. This extension of the dry land was brought about not only by the gradual accumulation of materials, but also by the upheaval of large tracts of stratified deposits; for, though the loftiest mountain-chains did not yet exist, ranges like those of the Alleghanies and the Jura belong to this division of the world's history. During this time, the general character of the animal and vegetable kingdoms was higher than during the previous age. Reptiles, many and various, gigantic in size, curious in form, some of them recalling the structure of fishes, others anticipating birdlike features, gave a new character to the animal world, while in the vegetable world the reign of the aquatic Cryptogams was over, and terrestrial Cryptogams, and, later, Gymnosperms and Monocotyledonous trees, clothed the earth with foliage. Such was the character of this second age from its opening to its close; and though there are indications, that, before it was wholly past, some low, inferior Mammalian types of the Marsupial kind were introduced,[2] and also a few Dicotyledonous plants, yet they were not numerous or striking enough to change the general aspect of the organic world. This age was throughout, in its physical formation, the age of large continental islands; while in its organic character it was the age of Reptiles as the highest animal type, and of Gymnosperms and Monocotyledonous plants as the highest vegetable groups.
There was an age in the physical history of the world when great ranges of mountains bound together in everlasting chains the islands which had already grown to continental dimensions,—when wide tracts of land, hitherto insular in character, became soldered into one by the upheaval of Plutonic masses which stretched across them all and riveted them forever with bolts of granite, of porphyry, and of basalt. Thus did the Rocky Mountains and the Andes bind together North and South America; the Pyrenees united Spain to France; the Alps, the Caucasus, and the Himalayas bound Europe to Asia. The class of Mammalia were now at the head of the animal kingdom; huge quadrupeds possessed the earth, and dwelt in forests characterized by plants of a higher order than any preceding ones,—the Beeches, Birches, Maples, Oaks, and Poplars of the Tertiaries. But though the continents had assumed their permanent outlines, extensive tracts of land still remained covered with ocean. Inland seas, sheets of water like the Mediterranean, so unique in our world, were then numerous. Physically speaking, this was the age of continents broken by large inland seas; while in the organic world it was the age of Mammalia among animals, and of extensive Dicotyledonous forests among plants. In a certain sense it was the age of completion,—the one which ushered in the crowning work of creation.
There was an age in the physical history of the world (it is in its infancy still) when Man, with the animals and plants that were to accompany him, was introduced upon the globe, which had acquired all its modern characters. At last the continents were redeemed from the water, and all the earth was given to this new being for his home. Among all the types born into the animal kingdom before, there had never been one to which positive limits had not been set by a law of geographical distribution absolutely impassable to all. For Man alone those boundaries were removed. He, with the domestic animals and plants which were to be the companions of all his pilgrimages, could wander over the whole earth and choose his home. Placed at the head of creation, gifted with intellect to make both animals and plants subservient to his destinies, his introduction upon the earth marks the last great division in the history of our planet. To designate these great divisions in time, I would urge, for the reasons above stated, that the term which is indeed often, though not invariably, applied to them, be exclusively adopted,—that of the Ages of Nature.