A first book in organic evolution

by D. Kerfoot Shute

A study of the fossils in the rock formations (see page 95) of different ages reveals the fact that thousands of species have lived and flourished in one age and then perished, never to appear in succeeding ages. A study of the fossils also reveals that life-forms have passed on from age to age, ever changing with the changing continent, some advancing to higher and higher levels, while others remained lowly.

The facts enumerated above may be instructively illustrated by a hasty reference to the history of organic life as unfolded by the rocks of different ages.[7]

Archæan Era. No evidence of life has been discovered in the Archæan rocks, but, inasmuch as with the dawning of the Palæozoic time, the waters of the sea were peopled with plants and animals living in great numbers and considerable variety, it is evident that the ancestors of these creatures must have lived during the Archæan ages.

TABLE OF STRATIFIED ROCKS AND THE SUCCESSIVE APPEARANCE OF TYPICAL ANIMAL LIFE-FORMS.

Cenozoic.	Quaternary.		Man, Mastodon, Saber-toothed Tiger and other Vertebrates. Invertebrates.
	Tertiary.	Pliocene.	Equus and other Vertebrates. Pliohippus. Protohippus. Invertebrates.
		Miocene.	Miohippus and other Vertebrates. Mesohippus. Invertebrates.
		Eocene.	Monkeys. Lemurs. Primitive Carnivora and Herbivora. Orohippus. Invertebrates. Eohippus. Fishes, Amphibians, Reptiles. Land Birds. Monotremes. Marsupials. Prim. Monodelphs.
Mesozoic.	Cretaceous.		Marsupials. Monotremes. Reptiles. Reptilian Birds. Amphibians. Teleosts and other Fishes. Belemnites and other Invertebrates.
	Jurassic.		Reptilian Birds, Reptilian Mammals. Rays, Chimæroids and other Vertebrates. Invertebrates.
	Triassic.		Primitive Marsupials, Primitive Monotremes, Reptiles, Amphibians, Fishes. Ceratites and other Invertebrates.
Palæozoic.	Carboniferous.	Permian.	Primitive Reptiles, Amphibians, Fishes. Invertebrates.
			Primitive Amphibians. Fishes, Invertebrates.
	Devonian.		Primitive Crossopterygii, Ganoidei, Dipnoi. Goniatites and other Invertebrates. Sharks.
	Silurian.	Upper.	Primitive Sharks. Ostracoderms. Invertebrates.
		Lower.	Blastids, Scorpions, Centipedes, Sea-urchins and other Invertebrates.	Invertebrates.
	Cambrian.		Marine Worms, Molluscs, Trilobites, Brachiopods, Crinoids, Star-fishes, Corals, Graptolites, Cystids, Sponges, Foraminifera.
Archæan.			Protozoans, though there is no evidence of life.|

Cambrian Period. The Cambrian rocks have furnished many different species of marine animals. Seaweeds are the only plants found fossil in the Cambrian rocks. Some plants and animals may possibly have dwelt upon the land, but if so they have failed to leave any record of their existence. The animals are all Invertebrates, but not of extremely low forms; they have progressed since their appearance in an earlier period. Siliceous Sponges are not uncommon. The Cœlenterates are represented by Graptolites and Corals. Echinoderms are rare, and are principally represented by Cystids, a very primitive type; true Crinoids and Star-fishes appear before the close of the period. That marine worms existed is indicated by borings and tracks in the sands, which have since consolidated into rocks. Mollusca burrowed in the mud or crawled over the ocean bed. Brachiopods existed, and Arthropods were represented by primitive types. The most characteristic of Cambrian fossils are Trilobites and Lampshells.

The former are extinct, and are not represented in the modern ocean; the latter, although greatly reduced in variety and numbers, are still found in various parts of the sea. During all this period, no backboned animals existed; then there were no fishes or amphibians, no reptiles or birds, and no mammals; nothing but invertebrates. The Cambrian fauna shows steady progress, being decidedly more advanced in the upper divisions than in the lower ones.

Lower Silurian (Ordovician) Period. The life characteristics of this period are very similar to those of the Cambrian. Brachiopods have developed extensively, though they have not yet reached their height of development. The Trilobites attained their greatest development in this period; but after the Lower Silurian these creatures commenced to decline in variety and numbers, and finally underwent extinction near the close of the Palæozoic era. In America no plants have been discovered above the grade of seaweeds, but a few of the higher Cryptogams are doubtfully reported in Europe. The character of the flora in a later (Devonian) period makes it highly probable that land plants were well advanced in the Lower Silurian period. Professor Scott states that the remains of land plants may be discovered at any time, though this must remain a matter of chance, inasmuch as all known Ordovician rocks are marine, and therefore not a favorable circumstance for the preservation of land plants. Foraminifera and Radiolaria were abundant in the seas of the Lower Silurian period. Sponges and Brachiopods are numerous and varied. Among Cœlenterates the Graptolites are very numerous and varied; the few and doubtful Cambrian Corals are succeeded by a considerable number of Ordovician genera and species. They were characteristically different from the reef-builders of modern seas. The Echinodermata have greatly increased in importance, being more numerous and varied. The Cystids reach their maximum development in this period. The Crinoids greatly increase in numbers and variety. The Star-fishes greatly expand; and a new and higher order of Echinoderms, the Sea-Urchins, make their first appearance in the later Ordovician period, but under very primitive forms.