Part 7

Chapter 73,087 wordsPublic domain

During the earlier ages (=Silurian= and =Devonian=) of this Palæozoic era, the place of the Appalachian chain of mountains was marginal sea bottom; but during the later ages (=Carboniferous=) it was, through repeated oscillations, in an uncertain state, being sometimes swamp land, sometimes covered with river sediment, and sometimes covered by the sea. It was during this _Carboniferous age_ that the great coal measures were formed; at this time also the climate was probably very uniform, warm and moist, loaded with carbonic acid gas and deficient in oxygen. This period was undoubtedly a paradise for the great coal-forming plants, but was very unsuitable for the hot-blooded air-breathing animals, such as mammals and birds, none of which existed at that period. Throughout all geological time the excessive amount of moisture in the air has been gradually removed by the growth of continents in size and height; also the superabundant carbon dioxide in the atmosphere has been removed in many ways, especially by the plants in the coal period appropriating the carbon. Many ages later, at the close of the =Jurassic= period, the Sierra Nevada range of mountains was uplifted. Up to this time the site of these mountains was a marginal sea bottom receiving vast amounts of sediment, and the Pacific coast-line was east of the site of the Sierra range. Naturally vast changes in physical geography and climate occurred in consequence. During these and the following =Cretaceous= ages that the continent was growing, the great interior Palæozoic sea and what may be called the Gulf of Mexico were more and more restricted, as shown in the map of North America in the Cretaceous period of its growth (Fig. 12). This great inland sea, separating the continent into an eastern and western portion, is now called _Cretaceous Sea_ instead of Palæozoic. This Cretaceous Sea covered the whole plains and plateau region of the continent, and extended from the Gulf of Mexico to the Arctic Ocean. At the end of the Cretaceous period of the continent this sea was obliterated by the gradual upheaval of this region and replaced by great lakes. At the same time the western marginal bottom of the sea was uplifted into the Wahsatch range of mountains; also at this time a line of islands in the Cretaceous Sea was uplifted into the Colorado mountains. All these events were entailing tremendous changes in physical geography and climate. Fig. 13 is a representation of the map of North America in the early =Tertiary= period, the time succeeding the Cretaceous period. In this period, the continent continuing to uplift, the lakes that occupied the site of the Cretaceous Sea are obliterated; the Coast Range mountains of California and Oregon are uplifted from marginal sea bottom (Fig. 13, dark shading); the Atlantic and Gulf borders are extended (dark shading), so that at the close of the Tertiary period the North American continent had attained its present form, except the southern portion of Florida and its keys. Since then the latter have grown and are still growing.

A later epoch still in the history of the globe is known as the =Quaternary= period, the period that immediately preceded our present epoch. The great features of this period, which is divided into =Glacial=, =Champlain= and =Terrace= epochs, are the wide-spread oscillations of the earth’s crust in high latitudes towards the north and south poles, attended with great changes of climate from temperateness to extreme cold.

The Glacial epoch was characterized by upward crust movements, the land becoming over one thousand feet higher than at present. The land was covered with ice, and an arctic severity of climate extended almost to the Gulf of Mexico. The Champlain epoch was characterized by a downward movement of the coast until it became five hundred feet or more below the present level, so that many lower portions of the continent became covered with sea. At this time there was a moderation of the temperature, a melting of the vast sheets of ice, and consequently a flooding of rivers and lakes, with many icebergs floating in them. The last or Terrace epoch of the Quaternary period was characterized by a crust movement up to the present condition of things.

What is true of the instability of the North American Continent is true of _all_ the continents of the globe. They have all grown from small beginnings to their present huge proportions, and are _now_ undergoing slow but irresistible changes. When these facts are held in mind, one may form a faint conception of the colossal changes that have taken place throughout the sweep of bygone ages. _Environment means a complexity of conditions almost infinite in their number and character, and almost infinite in their variations._

SECTION IV.

TRANSMUTATION OF LIVING FORMS.

It ought now to be understood that not only is the present environment changing, but also that it has been changing from the earliest geologic times. What, then, is to be said about the living creatures that have existed in the changing environment during all these geologic ages? Have they been rigid, unyielding forms? By no means! We know that they can be modified by altering the conditions at present; and a study of the fossils in the rock formations of the different ages of the world shows conclusively that animals and plants have altered in the past with the changing environment. The living creatures in the _Silurian ages_ differ from those in the succeeding _Devonian ages_, and these latter differ from those in the still later _Carboniferous ages_; and so on, to the present. Changing physical geography and climate are associated with changing forms in animal and plant life. The growing amplitude and complexity of a continent are associated with increasing complexity and specialization of its living forms. Just as the North American Continent of the Tertiary period differs from that of the Silurian ages, so also do the animal and plant forms of the Tertiary period differ from those of the Silurian ages. Just as there has been a continuity in the growth of the Silurian continent to that of the Tertiary ages, and the present, so, also, there has been a continuity of living creatures from Silurian to Tertiary and present times. Changing conditions of life have compelled modifications in living forms, and those creatures that were unable to adapt themselves to the altering conditions of life have perished, while those that did adapt themselves, through useful variations, lived and progressed in organization.

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.

=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.

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.

The true fishes soon developed to so great an extent in size and numbers that they swarmed in the Devonian seas and quickly became the rulers of the age; hence Devonian time is known as the =Age of Fishes=.

These early Devonian Fishes were of a more lowly organization than modern ones, and were what is called generalized forms; that is, they combined in themselves the characters of two distinct classes. They had distinct _amphibian characters_. From these generalized Fishes were afterwards formed, through many transitions, the _Amphibians_ and the _Fishes_ as branches from a common trunk. This illustrates what is a very general law, viz.: that the first-introduced examples of a class are not typical forms of that class, but intermediate forms or connecting links with other classes. Certain footprints recently discovered in the upper Devonian rocks of Pennsylvania indicate that the Amphibia, the lowest of air-breathing vertebrates, commenced their career in the latter part of the Devonian period; but they were inconspicuous among the monarch fishes.

=Carboniferous Age.= As in the Devonian Age so in the Carboniferous times, we find flowerless plants, such as ferns, horsetails, and club mosses; but in this coal-forming age they culminate, and have become gigantic in size, especially the two latter. At this time we again meet with the next higher order of plants, as cycads, pines, cypresses, etc.