II. SPECIAL CONTRIBUTIONS OF THE ORGANIC KINGDOMS.
An essential part of the historical chapters of the second volume will consist of the description and illustration of the life progress of the successive periods. It will suffice here to give a preliminary synopsis of the kinds of record made by the several groups of plants and animals.
A. Contributions of the Plant Kingdom.[290]
The record of plants in the early geological ages is extremely imperfect. In the very earliest times the conditions seem to have been wholly unsuited to the preservation of any relics of life; but even after animal remains were abundantly preserved in the sea sediments, the plant record was still very meager for a long period. This was probably due in the main to two chief causes: (1) the probable softness and perishability of the early types of vegetation, and (2) the fact that vegetation is preponderantly terrestrial. At no time has marine vegetation reached a high development. Land conditions favor decomposition, transportation, and erosion, and through these, destruction; and only under rather occasional and exceptional conditions did the old lands leave a good record of their life. Nevertheless all the great groups of plants, viz. the Thallophytes (algæ, fungi), the Bryophytes (mosses, liverworts), the Pteridophytes (ferns, horsetails, lycopods), and the Spermatophytes (gymnosperms, angiosperms) have left some record.
| Thallophytes (Thallus plants) | Algæ and algoid forms | Cyanophyceæ, blue-green algæ. | |
| Chlorophyceæ, green algæ. | |||
| Rhodophyceæ, red algæ. | |||
| Phæophyceæ, brown algæ. | |||
| Diatomaceæ, diatoms. | |||
| Coccospheres | Pelagic algæ(?). | ||
| Rhabdospheres | |||
| Charophyta, stoneworts. | |||
| Fungi and fungoid forms | Phycomycetes, algæ-fungi, water-molds. | ||
| Ascomycetes, ascus-fungi, mildews. | |||
| Basidiomycetes, basidium-fungi, mushrooms. | |||
| Æcidiomycetes, æcidium-fungi, “rusts.” | |||
| Schizomycetes, “fission-fungi,” bacteria. | |||
| Myxomycetes, “animal fungi,” slime-molds. | |||
| Lichens | Symbiont algæ and fungi. | ||
| Bryophytes (Moss plants) | Hepaticæ, liverworts. | ||
| Musci, mosses. | |||
| Pteridophytes (Fern plants) | Filicales | Filices, true ferns. | |
| Cycadofilices, cycad-ferns. | |||
| Equisetales | Equisetæ, scouring-rushes, horsetails. | ||
| Calamites. | |||
| Sphenophyllales. | |||
| Lycopodiales | Lycopodiaceæ, club-mosses. | ||
| Lepidodendra. | |||
| Sigillaria and stigmaria. | |||
| Spermatophytes (Seed plants) | Gymnospermæ (Naked seed) | Cordaiteæ, cordaites. | |
| Cycadales (cycads) | Bennettiteæ. | ||
| Cycadaceæ. | |||
| Coniferæ, evergreens. | |||
| Ginkgoaceæ, ginkgo. | |||
| Angiospermæ (Covered Seed) (Flowering plants) | Monocotyledoneæ, cereals, grasses, etc. (one-leafed seed). | ||
| Dicotyledoneæ, oaks, poplars, peas, etc. (two-leafed seed). | |||
The contribution of the Thallophytes (algæ, fungi, bacteria).—The Thallophytes embrace the simplest types of plants, and are probably the nearest present representatives of the ancestral forms. Some of them are minute one-celled organisms, as simple as an organism can well be conceived to be. The simple blue-green algæ of our fresh waters well represent this class. The most are, however, multicellular, and some (as the great seaweeds) rise to a degree of complexity and of a bodily segmentation resembling that of the higher plants. The various species are adapted to an extremely wide range of conditions; some live in hot springs at 170° Fahr., and some in Arctic seas at the freezing-point; some flourish in fresh water, some in brackish, some in salt water, and some even out of the water. This wide adaptation implies an ancient and plastic type. The fact that they flourish in waters so hot and sometimes also so sulphurous as to be fatal to most plants, suggests the possibility of their introduction during the very early volcanic stages of the earth, while conditions were yet uncongenial for other plants.
The geologic work of the thermal algæ is well shown in the beautiful travertine and sinter deposits of the Yellowstone Park (Figs. [215] and [218]). At the Mammoth Hot Springs the deposits are calcareous, while at most of the other hot springs silicious deposits are formed, in both cases partly, but not wholly, by the aid of algæ. The beautiful yellows, reds, browns, and greens of these springs are not mineral coloring, but living plants.[291] In the calcareous waters, the algæ are believed to cause the deposition of calcium carbonate from calcium bicarbonate by consuming the second equivalent of carbon dioxide that rendered the carbonate soluble.[292] In the silicious waters, the process of deposition is not understood. Similar deposits by the aid of algæ take place in the geyser regions of Iceland and of New Zealand, in the hot springs of Carlsbad, where they have been well studied by Cohn,[292] and in most other hot springs. The same, or very similar, forms of algæ abound in nearly all waters, fresh and salt, but the question whether they make calcareous and silicious deposits in notable quantity appears not to have received as yet the critical investigation its importance deserves, except in a few special cases. It is clear, however, that in the cool waters such deposits do not reach the conspicuous amounts that they attain in the thermal springs. In the shallow waters of the ocean, especially in the warmer regions, lime-secreting algæ are abundant and make large contributions to the lime deposits.
Among the higher algæ are the lime-secreting corallines or nullipores (Rhodophyceæ, red algæ), once regarded as animals, which contribute a notable part of the calcareous substance of coral reefs. They are important geologic agents in the temperate and tropical seas, and have been traced as far back in time as the early Paleozoic era.
The Challenger reports[293] describe two forms of minute calcareous spherical organisms, Rhabdospheres and Coccospheres, as very abundant in the surface-waters of the temperate and tropical seas, and as important in contributing to the calcareous deposits of the sea-bottoms. The affinities of these bodies are in doubt, but they are regarded by Murray as probably pelagic algæ.
The stoneworts (Characeæ), an aberrant group of algæ inhabiting fresh and brackish water, secrete notable quantities of calcium carbonate in and around their tissues, and the accumulation of these gives rise to marl or limestone. It has recently been urged that our so-called shell-marls are mainly due to Charæ,[294] the molluscan shells being incidental rather than essential constituents.
In very ancient and also in some of the later strata, there are limestones that do not carry any visible fossils, and their origin is, therefore, debatable. There are also not a few limestones that are made up of a fine-grained base through which are scattered molluscan shells, corals, etc., in a fine state of preservation. The condition of these fossils bears rather adversely on the view that shells, etc., have been powdered in sufficient numbers and to a sufficient degree to form the compact base. In all these cases the usual explanations leave something to be desired. It is worth considering whether low forms of plants may not be among the undemonstrated agents in forming these apparently unfossiliferous limestones or parts of limestones. The calcium carbonate deposited by the algæ is in minute and delicate form, and is usually crystalline while yet in the living tissues. It is, therefore, easily subject to comminution and to such further crystallization as would obscure the minute features that constitute the evidences of algal origin.
The more complex and conspicuous algæ, the seaweeds, have left impressions of their stems and fronds on the marine beds of most of the periods, but they are usually obscure. Seaweeds are perhaps the source of the vegetal matter in certain carbonaceous shales and limestones. As seaweeds extract bromine and iodine and certain metallic ingredients from the sea-water, some of the iodine and bromine springs issuing from ancient marine deposits, and certain ores, may owe their origin to ancient seaweeds.
Diatoms, minute plants of the Thallophyte group, secrete a delicate framework of silica which becomes a contribution to the silicious deposits. Diatoms have sometimes contributed the material for very considerable beds, such as those of the ooze-bogs now forming in the marshes of the geyser basins of the Yellowstone Park,[295] and the diatom oozes of the deep sea ([Fig. 353], [p. 425]).
Fungi, for obvious reasons, have left but scant traces of themselves.
Bacteria are believed to be recognizable as far back as the Paleozoic era. They are now the chief agents in the decomposition of organic matter, and may be regarded as the prime enemies of the fossil record. It is probable that similar decomposition took place actively in the earliest ages, for otherwise the remains of the ancient organisms should be more abundant. There is hence a theoretical probability that bacteria flourished as far back as the stratigraphic record goes. Not unlikely they were originally simple algæ that turned from the primitive habit of making their own food, to living on other organisms or their remains, and in so doing lost their power of manufacturing chlorophyll and of using inorganic carbon compounds. Their remarkable adaptation to the most varied conditions, and their extraordinary ability to endure the greatest vicissitudes of environment, support the view that they are a very ancient and plastic form.
At present certain bacteria are important to higher vegetation because of their ability to use the free nitrogen of the atmosphere and to combine it into forms available for the higher plants. It is not improbable that they have subserved this important function through all the known ages. Some experiments seem to show that certain of the existing algæ have this power, and possibly the ancestral forms of plants possessed it. The bacteria, being a derived and not an original form, could not have performed the function for the first plants. It is possible, of course, that the inorganic supply of nitrogen compounds was sufficient for plant life at the outset.
The contribution of the Bryophytes (liverworts, mosses).—The mosses and liverworts have left no certain record of their work in the earlier and middle geologic eras, and, if they existed at all, their contributions were unimportant. Although low forms of plant life, they are not primitive ones, as they are characterized by a definite alternation of generations implying a considerable time antecedent to the attainment of their present forms; hence there are no very cogent theoretical reasons for assigning them a place in early geologic history, though their absence cannot be affirmed. Some botanists think the Pteridophytes were derived from some ancestral form of liverwort, which, if true, would require the presence of the latter in an early geologic period; but the negative geological evidence relative to their presence favors the alternative view that the Pteridophytes were derived from some form of the Thallophytes by an independent line. In recent times, certain of the mosses, especially the sphagnum mosses, have played a notable part in the formation of peat accumulations. For this, their habit of growing in bogs, and of dying below while they continue to grow above admirably fits them.
The contribution of the Pteridophytes (ferns, horsetails, lycopods, Sphenophyllum).—The Pteridophytes include the most important fossil plants of the earlier and middle geologic eras. To them we owe chiefly the great carbonaceous deposits of the Coal Measures and probably most of the disseminated carbons of the early and middle eras; perhaps also much of the natural oil and gas. Their special work is so conspicuous that it will be noted at length in the chapters on the Devonian and Carboniferous periods, and hence may be passed here with brevity. The ferns, now known more for their beauty than their importance, are the representative type of the group, and are really a wonderful family, having preserved their characteristic leaf-forms with a persistence attained by no other group of plants. The Paleozoic ferns are recognizable as such by every one, irrespective of botanical knowledge; indeed it is the detection of the differences, rather than the resemblances, between the ancient and modern forms, that requires expert knowledge. This continuity shows that since their introduction the changes of climate have never been so great as to prevent their propagation, without radical modification, in some part of the globe, and this fact rather narrowly limits the range of surface temperatures, and of other climatic vicissitudes. The persistence of the Equisetæ (horsetails, scouring-rushes) and the lycopods (club-mosses) bears like testimony, as does the persistence of life in general; but the rather delicate ferns are perhaps more obviously significant than most organisms.
The contribution of the Spermatophytes (seed plants, including gymnosperms or “evergreens” and angiosperms or “flowering plants”).—The angiosperms, the dominant group to-day, make their appearance in the record in the latter part of the Mesozoic era, and their contribution is, therefore, relatively modern. They contributed to the coals, lignites, oils, and organic gases of the late geological periods, as did the Pteridophytes in the earlier periods, the latter participating, however, in the late deposits. Perhaps the most important function of the Spermatophytes lay in their superior serviceability as food for the higher land animals, by virtue of their seeds, fruits, and foliage. Neither the Thallophytes, Bryophytes, nor Pteridophytes, nor all combined, approach the Spermatophytes in food value for the higher types of animal life, and it is doubtful whether the higher evolution of the land animals could have taken place without the previous introduction of the seed plants. It will be noted in the historical narrative that the great placental group of mammals came in and deployed with marvelous rapidity, as geological progress goes, soon after the Spermatophytes became the dominant form of vegetation.
Plant life terrestrial rather than marine.—It is to be noticed that the chief development of all the great groups of plants took place on the land, or in the land-waters, rather than in the sea. This is preeminently true of the higher types, and appears also to be true of even the Thallophytes, although the number of individual algæ and their total mass is very much greater in the sea than on the land and in the land-waters. But the fresh-water algæ appear to possess in a higher degree than the marine forms those plastic and germinal characters from which new forms spring, and are probably to be regarded as the parental type. These are facts to be pondered on, since it has been the current opinion of geologists that life arose in the sea and was propagated thence to the land. The alternative view that life developed primarily on the land and in the land-waters and migrated to the sea is not, however, without its support in the plant world, as we thus see, and the plant world was the primitive one; the dependent animal world necessarily followed its development. The hypothesis of a terrestrial origin of life throws a very suggestive cross-light on many geological problems, as will be seen later, and it may well be entertained as an alternative working hypothesis until the facts are more fully developed.
B. Contributions of the Animal Kingdom.[296]
As already noted, animal life is dependent on the decomposition of matter organized by green plants, and the conversion of its potential energy into active forms. Animals are, therefore, dynamic rather than constructive agencies. Nevertheless they transform organic vegetal matter into organic animal matter, and this is sometimes really an advance in organization. The organized animal matter is subject to preservation in some small degree, though it usually perishes. Some contribution is, therefore, made to the organic deposits, chiefly in the form of hydrocarbons. It is the view of some geologists that the natural oils and gases have an animal origin in the main.
| Protozoa The simplest animals) | Rhizopoda | Foraminifera. | |
| Radiolaria. | |||
| Flagellata | Unknown in fossil state. | ||
| Infusoria | |||
| Gregarina | |||
| Cœlenterata (Sponges, corals, jellyfishes) | Porifera | Spongiæ | Calcareous sponges. |
| Silicious sponges. | |||
| Cnidaria | Anthozoa, coral polyps. | ||
| Hydrozoa, hydroids and medusæ. | |||
| Echinodermata (Crinoids, starfishes, sea-urchins) | Pelmatozoa | Cystoidea, cystids. | |
| Crinoidea, stone lilies. | |||
| Blastoidea, blastids. | |||
| Asterozoa | Ophiuroidea, brittle-stars | ||
| Asteroidea, starfishes. | |||
| Echinozoa | Echinoidea, sea-urchins. | ||
| Holothuroidea, sea-cucumbers. | |||
| Vermes (Worms) | Platyhelminthes | Rare as fossils. | |
| Rotifera | |||
| Nemathelminthes | |||
| Gephyrea | |||
| Annelida, sea-worms. | |||
| Molluscoidea (Mollusc-like forms) | Bryozoa, sea-mosses. | ||
| Brachiopoda, lamp-shells. | |||
| Mollusca (Molluscs) | Pelecypoda, lamellibranchs, bivalves. | ||
| Scaphopoda, tusk-shells. | |||
| Amphineura, chiton. | |||
| Gastropoda, univalves, snails, etc. | |||
| Cephalopoda, nautilus, cuttlefish. | |||
| Arthropoda (The articulates) | Branchiata | Crustacea. Trilobita, trilobites. Gigantostraca, horse-shoe crabs. Entomostraca, ostracoids, barnacles. Malacostraca, lobsters, crabs. | |
| Tracheata | Myriapoda, centipedes. | ||
| Arachnoidea, spiders, scorpions. | |||
| Insecta, insects. | |||
| Vertebrata | Cyclostomata, lampreys. | ||
| Pisces (fishes) | Selachii, sharks. | ||
| Holocephali, spook-fishes. | |||
| Dipnoi, lung-fishes. | |||
| Teleostomi, ganoids and teleosus. (common fishes). | |||
| Amphibia, amphibians, batrachians. | |||
| Reptilia, reptiles. | |||
| Aves, birds. | |||
| Mammalia (mammals) | Prototheria, monotremes. | ||
| Metatheria, marsupials. | |||
| Eutheria, placentals. | |||
As dynamic organisms animals have need for supporting- and working-frames, for protective covering or housing, and for offensive and defensive weapons, and these have been constructed chiefly out of inorganic matter, and subordinately of indurated organic matter. It is through these that animals have made their chief contribution to the material of the geologic record. Skeletons and other hard parts to give internal stiffness or firmness; shells, plates, indurated integuments, and various other forms of external protection; teeth, spines, horns, and other means of gathering and masticating food, and of attack and defense, contribute material to the deposits, and form a record of the life activities and of the physiographic environment. All of the eight groups of animals, viz. Protozoa, Cœlenterata, Echinodermata, Vermes, Molluscoidea, Mollusca, Arthropoda, and Vertebrata, have left some record, but it is in all cases a very imperfect one.
The contribution of the Protozoa.—The Protozoa are related to the animal kingdom much as the Thallophytes are to the vegetable, and the two bear a close structural resemblance to one another. So near, indeed, do the Protozoa and the Thallophytes approach one another in their minuteness and simplicity, that the place of not a few organisms is in doubt, and the two kingdoms, in general so different, seem here to blend in the group Flagellata. The Protozoa are usually very minute one-celled organisms with very little differentiation of tissue or organs. Of the four classes of Protozoa, only one, the Rhizopoda, is found in the fossil state. The rhizopods secrete silicious skeletons, and calcareous, silicious, and chitinous tests of a great variety of forms, and this gives them geologic importance. The deep-sea oozes and the chalk deposits are their best-known contributions at present. They have probably played a more important rôle in the formation of ordinary limestones and silicious silts than can be demonstrated, because of the delicacy of their relics and the ease with which these are pulverized by wave-action in the shallow seas, or changed by recrystallization or by concretionary aggregation. The globigerina oozes are formed largely from the calcareous shells of Foraminifera ([Fig. 351]), one of the orders of rhizopods, among which the genus Globigerina is a leading form. Those forms which make the deep-sea oozes live, not on the bottom, but near the surface of the open sea, and on the death of the organisms, the shells, tests, and skeletons sink to the bottom. Chalk is formed in a similar way from calcareous Foraminifera, but not necessarily in very deep water. Foraminifera live in shallow water as well as in the open sea, and in this case they sometimes creep on the bottom or are attached to algæ, but their deposits in shallow water are usually much obscured by other kinds of deposition and by destructive action. Some of the foraminiferal shells are divided into chambers and assume various spiral forms, of which the Nummulites, named from their resemblance to coins, are notable examples. These formed an important part of the nummulitic limestone of the Eocene period.
The radiolarian ooze is characterized by the silicious tests of various members of the silica-bearing order, Radiolaria. The “Barbadoes earth” and “Tripoli” are notable deposits of fossil radiolarians.
The contribution of the Cœlenterata.—The Cœlenterata embrace the sponges, the coral polyps (Anthozoa), and the hydroids and medusæ (Hydrozoa). The contribution of coral polyps to the formation of limestone is most important, and is too familiar to require elaboration here. The corals range throughout nearly the whole fossiliferous series, and their development will be followed and illustrated in the historical chapters.
The sponges are widely represented by their spicules, and not uncommonly their aggregate form is preserved even in very ancient strata. Their contribution is largely silicious, but is partly calcareous. The hydroids and medusæ have left little trace of themselves in the rocks, although impressions supposed to represent medusæ are found in strata as early as the Cambrian. Certain coral-like forms, as the Millepores, Tubularia, and Stromatopora, are classed as Hydrozoa. The graptolites, delicate leaf-like floating forms, very serviceable in marking exact horizons on different continents because of their free distribution, are also classed here.
The contribution of the Echinodermata.—Under the echinoderms are grouped the crinoids (sea-lilies), cystoids, blastoids, ophiuroids (brittle stars), asteroids (starfishes), echinoids (sea-urchins), and holothuroids (sea-cucumbers). This is one of the marked groups of ancient as well as modern life, and its beautiful fossils grace every period in which life relics are well preserved. The cystoids and crinoids, and later the blastoids, were prominent in the Paleozoic ages, while the remaining forms were more conspicuous later, though early introduced. All divisions, except the holothuroids, whose softness prevented, have left a good record, as fossil records go. Their relics are chiefly calcareous, and they most abound in the limestones, some of which are largely made up of their remains, as the encrinital limestone ([Fig. 349]). They will be subjects of frequent comment and illustration in the historical chapters.
The contribution of the Vermes.—Most of the worms are ill adapted to fossilization and are not known in the fossil form. The segmental worms of the sea, the annelids, however, left some traces of themselves in tubes and borings and in tracks and sometimes by fossil jaws and teeth. They range from the earliest fossil-marked horizons onward, but seem to have always been an inferior group.
The contribution of the Molluscoidea.—This group includes the bryozoans, whose fossil products closely resemble the minute-celled corals, and the brachiopods, whose shells closely resemble those of the molluscs. Both are calcareous and make important contributions to the formation of limestone ([Fig. 350]). A few brachiopods secrete calcium phosphate instead of calcium carbonate. Both classes have a great geologic range and their fossils are valuable aids in identifying and correlating formations. Probably the brachiopods are more utilized for this purpose than any other single class. They are the symbol of conservatism and persistence, ranging from the Cambrian to the present time, and embracing some forms that have scarcely changed to the extent of generic difference in that time.
The contribution of the Mollusca.—The molluscs have also ranged from the earliest well-recorded times, and some divisions, as the pelecypods (lamellibranchs, embracing clams, oysters, etc.) and gastropods (snails, etc.), have undergone no very marked change beyond a rather ample and progressive development; but others, as the cephalopods (nautilus, squids, cuttlefish, etc.), mark out the progress of the ages by distinct and striking changes of form. Their shells are chiefly calcareous and they have contributed materially to the formation of limestone. Muddy and sandy bottoms are, however, more congenial to the pelecypods and gastropods than to the corals, crinoids, and many other limestone-forming types, and hence fossils of these molluscs frequently abound in shales and sandstones and give them a calcareous element. In sandstones, however, the calcareous matter is often dissolved out and only the casts of the shells remain. The molluscs will be much cited and illustrated in the historical chapters.
The contribution of the Arthropoda.—This group embraces the crustaceans, myriopods, spiders, and insects. The hard parts of their bodies are mainly horny or chitinous forms of organic matter, and hence their relics differ notably from the inorganic calcareous and silicious remains of most of the preceding forms. The Arthropoda did not at any time form a notable stratum of rock. Their geologic value lies chiefly in what they teach of the progress of life and its relations, and the aid they render in correlation and identification. In these respects the group is a notable one. It was represented in the early fossiliferous strata by the trilobites, one of the most interesting of all types of fossils. These were probably the most highly developed organisms of their times and give the clearest hints of the stage of psychological and sociological development that had been reached when first the record of life is opened to us. The record of the myriopods, spiders, and insects dates from the middle Paleozoic, and gives the first clear hints of animal life on the land.
The contribution of the Vertebrata.—In the vertebrates the dynamic or working organism may be said to reach its highest expression, unless it be in the flying insects, and their inorganic residue becomes relatively unimportant in rock formation. Although the greatest of all animal types in most respects, it has never formed more than trivial beds of rocks. There are occasional “bone beds,” but they are thin and limited in extent, and only partially formed of vertebrate matter. The geological importance of the vertebrates lies in the higher field of life evolution and in its mental accompaniment. Fishes excepted, the vertebrates are mainly land types, and have for their chief colleagues plants and insects. The other groups of animals are mainly, though not wholly, marine. The vertebrates have little place in the Paleozoic record, except near its close, but they dominate the Mesozoic and Cenozoic eras, and are conspicuously the master type to day.