A first book in organic evolution

by D. Kerfoot Shute

Among Arthropods the Trilobites increase greatly in numbers and variety, and attain their maximum development in this period. The occurrence of a Centipede in Ordovician rocks indicates the interesting fact that terrestrial animal life had already begun.

One of the most striking differences between the Lower Silurian and Cambrian periods is the great advance made by the Molluscs, in variety and numbers, during the former period. Molluscs exhibit the most significant change in the great expansion of the Cephalopods, a few of which had appeared in the uppermost Cambrian rocks. In the Lower Silurian period the Cephalopods became one of the predominant types in the marine life. In modern times nearly all Cephalopods are naked (Cuttlefish and Squid), only a few having a shell (Nautilus and Argonaut). The naked forms are higher creatures than those with shells. In the Lower as well as in the Upper Silurian periods no naked forms existed, but only the lower forms with shells (Nautiloids). In modern times and throughout later geological periods only Nautiloids with coiled shells were in existence. In both of the Silurian periods the shells were all straight, and the animals were called Orthoceratites. Such animals were extremely abundant in those times, and often reached an enormous size. Specimens have been found which were ten inches in diameter and over fifteen feet long. They were the most formidable animals of those early ages, and were the rulers and scavengers of the seas; so that the Lower and Upper Silurian periods are known as the Age of Molluscs.

Upper Silurian Period. In this period great progress is made in the history of life on the globe. Upper Silurian life is the continuation and advance of the organic system that flourished in the Ordovician, certain groups expanding, others diminishing; and some new groups now appear for the first time. Sponges are still common. Among Cœlenterates the Graptolites have greatly diminished; Hydroid Corals have become important features of the seas and in the formation of the reefs. True Corals increase largely and play a more important part than in the preceding period. Honeycomb and Chain Corals are quite characteristic of this period.

Among Echinoderms there is a diminution of the Cystids, and a marked increase of the Crinoids. A new class of Echinoderms, the Blastids, now make their first appearance. The Sea-urchins and Star-fishes have increased in variety and abundance. Brachiopods still exist in multitudes, but under changed forms. Among Molluscs the Orthoceratites are still the rulers of the seas. Among Arthropods the Trilobites are still numerous, though decidedly less so than in the preceding period. The land animals are insects of low types, mainly allied to cockroaches. Scorpions are also present as fossils in the Upper Silurian rocks. These animals prove the existence of a contemporaneous land vegetation.

During all the many millions of years that constitute the Silurian periods, hosts of species lived and died; hosts of variations were induced in the living creatures by the ever-changing environment; so that at the close of this time the Silurian rocks show that some of the life-forms, lowly as they were, had yet climbed higher in the scale of organization.

The later Silurian rocks reveal the remains of an entirely new branch of living forms, a higher branch of animals (Vertebrates) than had ever before existed. These remains are those of Ostracoderms and primitive Sharks. But at that early time they held a very subordinate position among the hosts of living creatures; the Molluscan Orthoceratites were still the rulers of the seas; it was still the Age of Molluscs.

Devonian Period. The next set of ages succeeding the Silurian time is known as the Devonian period. Many of the kinds of creatures living in the Silurian ages are also found in these ages, but under changed forms. The Devonian seas had a great abundance and variety of Corals and Crinoids. The Chain Corals have perished, but the Cup and Honeycomb Corals still live on with modified appearance. The Graptolites are almost extinct. The Orthoceratites still live on, much reduced in numbers and size. The Molluscan Cephalopods have been revolutionized. New forms have grown out of the variations of the past, so that now in Devonian times we have the introduction of the great Molluscan Ammonite family, under the forms known as Goniatites, which are characteristic of this and the succeeding Carboniferous ages. The Trilobites still continue under new forms, but much reduced in size and number. Brachiopods are abundant and diversified.

In this age, for the first time, land plants become conspicuous. The Devonian forests consisted of the highest flowerless plants, such as ferns, horsetails, and club mosses; and also the lowest flowering plants, such as cycads, pines, cypresses, etc.—plants which have imperfect and inconspicuous flowers. In all ages, as now, land vegetation has been closely related with insects. Insects, though rare as fossils, are found in connection with the forests in the Devonian period. They are among the lower orders of the class, and are somewhat allied to cockroaches and dragon flies.

The most characteristic feature of the Devonian age is the expansion of that new and higher class of animals, the backboned animals; for during the earliest Devonian or latest Silurian times the Fishes made their appearance. At first many of them were comparatively small in size, they were few in numbers, and of strange, unfishlike forms. Such were the Ostracoderms, which, though generally called fishes, belong to a type much below the true fishes, and more nearly allied to the Lampreys, for they were devoid of jaws and paired fins. There was a great variety and wealth of true fishes in the Devonian period. The Elasmobranchii (Sharks) were well represented, though very generalized ones. Crossopterygii, Dipnoi and Ganoidei were important elements of the fish fauna of this period. At this period there was an entire absence of those highly specialized fishes (Teleosts) which in modern times make up the vast majority of fishes, both marine and fresh-water.