The science of life : fully illustrated in tone and line and including many diagrams

BOOK 3

pages all tidily in order, the fossil-writing abundant and easy to interpret. In most cases these confusions have been analysed and overcome, and the result is that, with rare exceptions, the. fossil-bearing rocks of the world can now be assigned to their proper position in the time-scale—their right place among the pages of the Book of Earth.

As a result, the earth’s history has been divided up, as this book is divided into books, chapters, sections, and so forth, into subdivisions of various grades. Two sets of terms are used, according as we are thinking in terms of geological time or in terms of layers of rock. The main divisions of geological time are usually called Eras ; within each Era a number of Periods (or Epochs) are distinguished, and they are further divided into sub-periods.

When, on the other hand, we are speaking of rock-layers, we refer to a System as our main unit, roughly corresponding to the rocks laid down during one Period of time. The Systems are divided into Formations, and so on down to the narrow Zones, which may be likened to the paragraphs of a book. The Cretaceous System is thus an actual set of rock layers laid down during the time of the Cretaceous Period.

For our purpose, the Eras and Periods are all we need trouble about ; when we need to go into further detail, we can specify subperiods by simply using the words “ upper,” “middle,” or ‘‘ lower.” For instance, the Carboniferous Period, during which the world’s coal was deposited, is the fifth of the six Periods of the third main Era. The Upper Carboniferous, then, is its latest sub-period, for obviously the uppermost layer must have been the last deposited.

‘The names of the Periods are at present, unfortunately, unfamiliar to the majority of people. ‘They should be as well known as the names of the continents and main countries of the world, or as the great dates of human history. We give a set of diagrams here, geological time-maps (Fig. 122), and we suggest that the reader make himself familiar with the divisions of these diagrams if he does not know them already. Very roughly the divisions of these diagrams are spaced out in the proportion of time assigned to each Period. We shall note later in this Book how the lengths of these Periods have been determined.

Now, the fact of primary importance in the history of life displayed by these geological Periods is the orderly succession of living forms. ‘They progress. They progress from 196

THE SCIENCE OF LIFE

GEAR i ER “3

simple beginnings to more complex and versatile types. At the bottom (earliest) of our rock series come rocks with barely a trace of life and then in succession life unfolds. Comes first the ARcHEOzoIc ERA with the dawn of life. Then the Prorerozoic Era, with creatures as highly organized as worms. Then the vast PatEozorc Era. There are no vertebrata at all and no evidence of land life in its opening period, the CampBrian. Then in a second period, the Orpovicran, is the dawn of vertebrate life.. Then comes the SILURIAN, in which fishes and some land plants and invertebrata appear. Then DEVONIAN and CARBONIFEROUS, with an everincreasing amount of land forms, and the whole of the era closes with PERMIAN, in which reptiles first appear.

Above these comes the Mesozoic System of rocks, that gigantic volume which tells of the Era of mighty reptiles and coniferous and cycad-like plants. Its formations (which like the Periods of the Paleozoic derive their names either from the districts in which the rocks are well-developed or from well-defined physical characters) are the Triasstc, the Jurassic, the CRETACEOUS.

Finally comes the Cenozoic Era, the age of modern life, of mammals, birds, grasses, flowering plants, and trees. And here we warn the reader of one of those exasperating indistinctnesses of nomenclature in which the scientific mind at its worst seems to delight. The Cenozoic Age is subdivided into the EocEeNne (dawn of recent life) and so onwards with progressive modernization of animals through the OLicocENE (slightly recent), MrocENE (less recent than the next division), the Prioc—ENE (more recent), the PLEISTOCENE (most nearly recent)—the Period of the last great Ice Age—and the “RECENT”? Period, since the retreat of the ice, in which we live. The use of local and mineral names for the formations is suddenly abandoned for these kindred confusable names. For some reason quite a number of slightly inattentive students get “mixed ”? with Miocene and Mesozoic, just as it is the commonest misapprehension in the world to substitute Paleolithic (a stage in human development) for Paleozoic. ‘The scientific systematist has never grasped what any novelist can tell him, that names must be distinctive if they are to be remembered. How would he like to struggle through a story in which Tompkins, Tomlins, Tomkinson, Robert Thompson, Robins and Robinson were the names of the principal characters ? Unhappily the present writers have no power to rechristen the geological formations, as