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.