CHAPTER XI.. The Geologic Functions of Life.

THE GEOLOGIC FUNCTIONS OF LIFE.

I. THE DISTINCTIVE FEATURES OF ORGANIC PROCESSES.

There is no reason to suppose that life processes, as we know them, were in operation in the earliest stages of the earth’s history. They were introduced and developed gradually during its progress. With life there came into the processes of the earth’s development three distinctive factors:

A. Certain chemical actions giving rise to compounds that are not known to occur independently of life.

B. Certain modes of aggregation of material, and certain kinds of bodily movements, not known except in association with life.

C. The mental element, under the direction of which certain new processes were inaugurated, and certain previous processes were modified and controlled.

+A. The Chemical Work of Life.+

The peculiar chemical phenomena connected with life chiefly concern the carbon compounds. In the inorganic world the carbon compounds are few and simple. In the organic world they become extremely numerous and complicated. These compounds are very unstable, for the greater part, and their partial decomposition gives rise to many additional compounds. Some of the true organic compounds and some of their decomposition products have the power of combining with inorganic substances, and so produce an additional series of semi-organic combinations. The total number of the compounds thus directly and indirectly connected with life greatly exceeds that of all inorganic compounds. Their mass, however, is very greatly inferior.

=Life material chiefly atmospheric.=—In the building up of the organic compounds, a necessary step is the decomposition of certain inorganic compounds. The chief of these is the carbon dioxide of the atmosphere and hydrosphere, the decomposition of which furnishes the carbon needed for the organic compounds. On this account carbon dioxide may be regarded as in some sense the basal material or the fundamental food of the organic kingdom, and hence it plays a radical rôle in the life-history of the earth.

Water, and the constituents of water, oxygen and hydrogen, play a larger part quantitatively, but a less distinctive part.

Nitrogen is also an essential element, and usually stands next to carbon, oxygen, and hydrogen in quantity.

These, it will be noted, are all atmospheric constituents, and the material of life is, therefore, dominantly atmospheric. This is even true of aquatic life, for it lives largely on the atmospheric constituents dissolved in the water. The function of life, considered from the material point of view, is not only fundamentally concerned with the atmosphere, and intimately dependent on its conditions, but its most important material effects appear to lie in its modification of the constitution of the atmosphere.

=The non-atmospheric factors.=—The atmospheric constituents are not, however, the only elements intimately connected with the life function. Compounds of sulphur, phosphorus, potassium, sodium, chlorine, iron, calcium, magnesium, silicon, and other elements are more or less essential to the life of many organisms, or are employed by them for their skeletons, coverings, etc. Incidentally, nearly all the common elements become intimately related to living organisms either in the relations of active elements in their physiological functions, or of passive elements in their structure or in their auxiliary parts.

Three Classes of Effects.

Out of life processes grow three rather distinct classes of results: (1) changes in the amounts and proportions of the constituents of the atmosphere and, to some slight extent, of the hydrosphere and lithosphere; (2) aid or hindrance to inorganic processes, such as disintegration, erosion, and deposition; and (3) distinctive products, either (a) of organic matter that would not have come into the existing combination but for life, such as peat, lignite, amber, etc., or (b) of special forms of inorganic matter that would not have arisen but for life, such as coral deposits, shell-marl, diatom ooze, etc.

(1) Changes in the composition of the atmosphere.

The succession of modifications which the atmosphere has undergone from time to time through the action of life will be discussed as the earth’s history is followed in the second volume. It may suffice here to note briefly the chief ways in which the atmosphere has probably been modified by the agency of life, not only as regards its quantity but also as regards the proportions of its constituents.

=The consumption and restoration of carbon dioxide.=—As the fundamental food of the organic world, carbon dioxide has suffered enormous consumption in the course of the geological ages, and is now reduced to the very small proportion of .0004 or .0003 of the whole. At the outset it was probably one of the most abundant constituents; possibly even the chief one. It has been partially restored, concurrently with its consumption, by animal respiration, by certain classes of plant action, and by combustion and other forms of inorganic combination. This restorative action has been incomplete at all known stages of the earth’s history, and hence there has been constant loss of carbon dioxide. The inorganic processes which have also profoundly affected both the consumption and restoration of carbon dioxide are here neglected and discussed elsewhere.

=The freeing and consumption of oxygen.=—The oxygen of the atmosphere is actively consumed by animals and by plants, but on the other hand, it is set free abundantly by green plants, and hence its amount has probably fluctuated from time to time according to the state of balance between the organic processes of its production, and those of its consumption. The consumption of oxygen by organic processes is, however, little more than a reversal of the previous process by which it was set free; for instance, green plants in forming their food set free the oxygen of the carbon dioxide used for the purpose. When the organic substance so formed is ultimately consumed through plant or animal action or by inorganic means, an equivalent amount of oxygen reunites with the carbon to again form carbon dioxide. And so if the whole of the organic matter is returned to the inorganic state, no more oxygen is consumed than had been before set free in the process of forming the organic matter. But, as a matter of fact, a large amount of organic matter has not gone back completely to the inorganic state, and this residue constitutes a factor of no small importance in the geological record.

=The organic residue.=—There is a certain portion of vegetation that is not consumed by animals or by other plants, and that escapes combustion and all kinds of ordinary decay, and this constitutes a part of the organic residue. Animals never completely oxidize all the organic matter they take into their systems; their bodies never entirely consume themselves. A like statement may be made respecting those plants that feed on organic matter. That which animals and plants leave unoxidized is indeed more or less preyed upon by other animals and plants, and relatively little escapes final reoxidation, but there is a remnant, and this constitutes another part of the organic residue. The more conspicuous forms of the organic residue are found in the mucks, peats, lignites, coals, organic oils, and gases, but in addition there is not a little disseminated organic matter in nearly all the sedimentary rocks; in the aggregate, this probably amounts to more than the distinct organic deposits.

=The meaning of the organic residue.=—All the unoxidized, or incompletely oxidized, carbon in the organic residue implies that oxygen has previously been separated from this residual carbon by plants and given to the atmosphere, and hence has been a source of atmospheric enrichment in oxygen. The amount thus contributed is equal to that which is required to restore the residual carbon to its original state of oxidation. So, in a similar way, the unoxidized hydrogen in the organic hydrocarbons and like compounds implies that oxygen has been separated from the hydrogen of water and given to the atmosphere, and hence this also is a source of atmospheric enrichment in oxygen. It seems safe, therefore, to conclude that the action of life, taken as a whole, has increased the free oxygen of the atmosphere.

While not here under consideration, it is not to be forgotten that inorganic processes involving the same atmospheric constituents have been in operation concurrently with the organic processes, and that they have also affected the amounts and proportions of the atmospheric constituents. Rocks have been oxidized in greater or less measure at the expense of the atmospheric oxygen, and hence when the total atmospheric problem is considered, there arises the question whether the amount of oxygen in the atmosphere has been increased or diminished during geological history, when the balance is struck between the inorganic and the organic actions. The probabilities seem to us to strongly favor the view that organic action has preponderated, and that the oxygen has been increased beyond its primitive amount, but that it has fluctuated during known geological history. The reasons for this view will appear in the historical chapters.

The disintegration of the crystalline rocks and the solution of limestone have consumed much carbon dioxide, and this is to be added to the loss through organic action. On the other hand, there are inorganic processes that supply carbon dioxide, and hence when the larger problem of the atmosphere is raised, the factors become so complicated that their consideration is best deferred to the historical chapters. This passing reference may stand us in good part lest we forget, for the moment, the inorganic factors in the atmospheric problem.

=The more inert factor.=—Nitrogen in the free state is relatively inert chemically, and it does not appear that it can be used directly by the higher plants and animals in appreciable amounts. Certain bacteria, and perhaps certain algæ and other low forms of plants, have the power of using free nitrogen, and this is a principal way in which it is put within the reach of higher plants. Nitrogen is also combined in small quantity in the atmosphere by electric action, and thus made available for plants. On account of the inertness of nitrogen and of the relatively limited amount required for organic purposes, the nitrogen of the atmosphere has been less consumed than the carbon dioxide. Besides this, the nitrogen compounds are very decomposable, and are very generally and completely returned to their original state. Deposits of nitrates or other nitrogenous compounds are relatively rare.

It is obvious that if there is any considerable source of supply concurrent with this slight loss, the amount of nitrogen in the atmosphere must have been increasing. We have seen that volcanoes give forth considerable quantities of nitrogen, and that this may be a real addition to the atmosphere, and not merely a return of the atmospheric nitrogen that had been carried down previously by underground-water. It has also been noted that crystalline rocks contain occluded nitrogen, which is doubtless freed by their disintegration. It is, therefore, not improbable that the nitrogen of the atmosphere has been increasing, both actually and relatively.

=Probable fluctuations of atmospheric composition.=—With this general sketch of the interplay of the atmospheric elements under organic influence, we are prepared for the further conception that if one or another of these actions was relatively more vigorous than usual for a period, it would bring about a variation in the proportions of the atmospheric constituents. If, for example, vegetation flourished luxuriantly for a long period, but was measurably protected from the organisms that preyed upon it and from inorganic decomposition, as by falling into water or by prompt burial under sediment, the atmosphere might be growing richer in oxygen. If, on the other hand, vegetation were being relatively reduced, as perhaps it is being reduced now by man, and if previous organic products were being reoxidized at an unusual rate, as they are now in the burning of timber, coal, natural oil and gas, the carbon dioxide of the atmosphere might be relatively increasing, while the oxygen might be relatively diminishing. The possible fluctuations of the atmosphere as the result of organic action are, therefore, matters of vital importance, and invite attention in the historical study of the earth and in the outlook into its future.

=The climatic effects of organic action.=—Interest does not, however, rest at this point. The researches of physicists have made it probable, if they have not altogether demonstrated, that the composition of the atmosphere has much to do with the climatic conditions at the surface of the earth. The atmosphere blankets the earth and equalizes its temperature. Acting as a screen, it subdues in some measure the intensity of the sun’s rays by day, while it retards the radiation of the earth’s acquired heat at night. This is in some measure the function of all the constituents of the atmosphere, but by no means of all equally. The oxygen and nitrogen are relatively diathermous, letting the sun’s rays pass in freely, and the earth’s rays pass out freely; but carbon dioxide and the vapor of water are much less diathermous, particularly to rays of low intensity, such as are thrown out by non-luminous bodies like the earth. It follows that while the solar rays come in rather freely and heat the surface of the earth, the dark rays which the earth radiates back are measurably arrested by the carbon dioxide and vapor of water, and serve to keep the air warm. The influence of the vapor of water is vividly shown in the different degrees to which cooling takes place at night in a dry and in a moist atmosphere, respectively, where other conditions are the same. Ice is said to form at night in desert regions where the air is extremely dry, even within the tropics, while in humid regions of the same latitude and altitude oppressively hot nights are common. The influence of the carbon dioxide is not thus familiarly demonstrated, since its amount varies but slightly in different localities, but physical experiment indicates that it has a similar function.

If the amount of carbon dioxide in the atmosphere varies from age to age, the climate of the earth must apparently vary accordingly, and on this is built one of the hypotheses of climatic variation subsequently to be considered. We shall find that there have been great changes in the climate of the earth during its history. There is good evidence of former glaciation, not only in the northern United States and in England, Germany, and central Russia, but in India, Australia, and South Africa. At other times, figs and magnolias grew in Greenland and Spitzbergen, and corals flourished in the Arctic seas. There is good evidence of arid periods where humidity now prevails, and of humid periods where aridity now prevails. It is not assumed that the influence of organic action on the atmosphere has been the sole, or perhaps even the main, cause of these great climatic changes, but it is believed that it has been an important contributing factor. It is even possible that the climate of the future is much dependent on the agency of man, as implied above, however little ground there may be to suppose that he will, with altruistic purpose, control his action with a view to its bearing on the generations that may live tens of thousands of years hence.

(2) Aid and hindrance to inorganic action.

=The promotion of disintegration.=—While the influence of organic action on the lithosphere is quite superficial, and far less radical than that on the atmosphere, it is still important. Plants promote both disintegration and disaggregation under certain conditions, and hinder them under others, as already set forth. Chemical action of a decomposing and solvent nature takes place in connection with the roots of plants, while their growth sometimes rends rocks into whose crevices they have insinuated themselves. The acids and other products of organic growth and of organic decomposition attack some of the constituents of the rocks and contribute to their solution and disintegration. On the other hand, organic matter entrapped in the sediments, and so introduced into the strata at various depths, often acts as a reducing agency, causing the deposit of substances carried in solution in the underground-waters. Ores are sometimes thus formed, as explained in the discussion of ore-deposits (p. 476). Organic action on the whole promotes solution and disintegration at the surface, and prepares the way for deposition below.

=Protection against erosion.=—Another important function of vegetation is the protection of the land surface against erosion, as already noted in the discussion of erosion. A mantle of grass, especially if it forms a turf, or a carpet of leaves protected by bush and forest, greatly retards surface wash. It does this not only because it directly covers the soil, but because it holds back the run-off and tends to prevent those violent floods which give to erosion its greatest intensity. There is a marked difference between the erosive work which a given amount of water will do if, in the one case, it runs off gradually, and in the other, precipitately. By way of offset, it is to be noted that the disintegrating action of vegetation prepares the rock material for easy erosion, and to this extent helps in its removal by the drainage; but on the average this is greatly overbalanced by the protection afforded by the vegetal covering, though this is not true in every instance.

=The influence of land vegetation on the character of the sediments.=—The presence or absence of a vegetal covering influences the kind of deposit which is derived from the land, particularly if the surface be occupied by crystalline rocks. If the surface be well clothed with vegetation, the crystals of the complex silicates, such as the feldspars, micas, and ferromagnesian minerals, are usually disintegrated into clayey products before they are removed, so that, when borne away and deposited, the result is common shale. Concurrently, the relatively undecomposable quartz-grains are rounded into sand, and deposited as common quartzose sandstone, while the calcareous material is borne away in solution and deposited as limestone. But if the surface be bare of vegetation, the crystalline rocks are usually disaggregated before they are decomposed, for destructive action works best at the junctions of crystals, and along cleavage lines, and hence the crystals are usually separated from one another before they are fully decomposed. In the absence of a covering to hold them in place until they are decomposed, they are apt to be washed away, and the resulting deposit consists in considerable part of grains of feldspar, mica, hornblende, and other minerals, which do not usually occur in well-decomposed sediments. The deposits are, therefore, of the nature of arkose, if the original rocks are granitic, or of the nature of wacke, as the term is used in this book, if they are of the basic type. On this is based the inference that a vegetal covering of the land extended as far back in the history of the earth as clay shales, quartzose sandstones, and limestones form the prevailing sediments.

(3) Distinctive deposits.

=Organic rocks.=—In the chapter on the origin and descent of rocks, a group of rocks formed directly from organic matter is recognized and described. The chief of these are peat, lignite, bituminous coal, anthracite, and graphite. It is the belief of many geologists that natural gases, oils, and asphalts are also mainly derived from animal and vegetal remains. An alternative view, advocated by Mendelejeff and Moissan, assigns the oils, gases, etc., in part at least, to deep-seated carbides to which water has gained access and developed hydrocarbons, after the analogy of acetylene. Whatever may be the truth relative to inorganic action, it is clear from geological conditions that some of the natural gases and oils are organic products. Besides the more common organic deposits, there is a long list of minor products, among which are amber, copalite, paraffine, ozocerite, camphene, etc. Guano and coprolites represent the excrementitious class.

=Inorganic rocks due to life.=—Besides these deposits of organic matter, or of its decomposition products, there is a large class formed from the inorganic matter that served auxiliary functions in the economy of life, such as shells, skeletons, etc. For the greater part these are composed of calcium carbonate, and give rise to limestones, marls, chalk, etc. Not a few, however, are silicious, and give rise to flints, cherts, and silicious earths. Some are formed of calcium phosphate, and a few of other inorganic material. The deposits formed in these ways have been defined in the chapter on rocks.

Fossils.

The term fossil is used so comprehensively as to include not only the remains of plants and animals themselves, but their tracks, impressions, casts, replacements, and all other distinct traces. It also embraces nests, borings, implements, and other distinctive products. These enter into the formation of the two classes of rocks just considered, but they have an independent function. They constitute the specific record of life, and their study not only reveals much of the past history of plants and animals, but furnishes one of the most important means by which the ages of formations are determined. In the early development of the science it was found that the uppermost and hence the latest beds of rock contain fossil forms either identical with those now living, or closely similar to them; that beds below these bear life relics that depart somewhat more from the living forms, and are somewhat less highly developed; that beds still lower bear fossils that depart still more from the living types, and are more primitive in general, and so on down as far as fossils are found.

=The general order of life succession determined by stratigraphy.=—Thus it appeared from the evidence of the strata that there was a general order of life succession. It was also found that this was, in its main features, the same for all the continents. By continued and close studies, the particulars of the succession were worked out more and more fully, and the work is still being pushed forward to greater and greater degrees of refinement. At the same time, it was found that there were different faunas and floras in different parts of the world in past times, much as there are now; that there were shiftings and migrations as now; that given species were increasing in some regions and dying out in others, and that innumerable variations and complications entered into the evolution and distribution of the life forms. But under and through all these there run a sufficient number of common features to show beyond reasonable question the order of succession of life.

Throughout all this study, the chief guide was the actual order in which the fossils were found in the succession of strata, because there is no evidence so conclusive of the order of events as the superposition of the sedimentary beds when they are normal and undisturbed. By the study of the fossils in the successive beds, it was found that there was a more or less progressive evolution of plants and animals brought about by modifications of their forms, and that these modifications assisted in determining the order of succession when the evidence of the strata was defective; and so the biological and stratigraphical factors reacted helpfully on each other.

=Fossils as means of correlation.=—While stratigraphy was thus, in the earliest stages, the main reliance in determining the order of events, and biology was the chief gainer, in the end stratigraphy received ample compensation, if indeed it did not become the greater beneficiary; for at no known and accessible place is there a complete succession of sedimentary beds. There are great series here and there, but their connections with one another are more or less concealed by surface formations or water-bodies. So also at many places the stratified series has been broken up by deformation, or cut away by erosion. Hence there was need for some reliable means of matching the beds of separated series, and of making up a complete ideal series. This means is found in the fossils they contain. While the variations of the faunas and floras in different regions, and their migrations, introduce some minor difficulties, the relations of the fossiliferous beds of one region to those of another can be determined with great satisfaction, and often with great precision. This is particularly so when abundant floating or free-swimming species lived in the seas and were freely fossilized, for they were deposited on the coasts of all the continents at practically the same time, and no uncertainties from migration or local differences in rate of evolution intervened to throw doubt upon the correlation. Without the aid of fossils, the correlation of the deposits on the separate continents would be attended with grave obstacles and much uncertainty, if not with quite prohibitive difficulties.

+B. Special Modes of Aggregation and of Movement.+

Inorganic solid matter is chiefly crystalloidal; organic matter is chiefly colloidal; but there are colloidal states of inorganic matter and there are crystalloids among the organic products. In the inorganic world, solids very generally tend to organize in the form of crystals; in the organic world, they as generally tend to organize in the form of cells. Neither tendency is complete or exclusive, but each is dominant in its own sphere.

Still more distinctive than the formation of cells is the growth of complex organized bodies, the differentiated members of which perform special functions for one another, and are mutually dependent on one another. This is a profound departure from the habitual modes of the inorganic world.

Still more so is the power of voluntary motion in more or less disregard of outside physical influences. Through this power, distribution may take place contrary to current and wind, and to gravitation itself. From the view-point of past geologic transportation, this is perhaps more singular than important, for no great mass of matter has been transported contrary to the influences of gravity, wind, and current, by the exercise of this peculiar power of animals, but it is not without geologic importance in the migrations and in the redistributions of organic influences that arise from migrations. When the influence of man is included, the geologic effects require consideration, but here the third distinctive factor, the mental element, comes into effective play, and we pass to its consideration.

+C. The Mental Element.+

Current opinion does not recognize a mental element as residing in the plant world, and it is divided as to the degree of its development in the lower animal kingdom, but its influential presence in the higher animal orders and in man is beyond legitimate question. Two phases are to be recognized: (1) the material work done under the stimulus and direction of mental impulses, as, for example, excavations, transportations, changes of drainage, removal of forests, cultivation of soil, etc., and (2) the intellectual work of the faculties themselves irrespective of material changes. In one view, geology is a purely material science concerned solely with the formation of the earth and with the physical development and relations of its inhabitants. In another, geology is a comprehensive historical science concerned with every phase of the world’s history, and certainly not least with the higher forms of life development, with their psychological, sociological, and other phases of mental attainments, since these are the highest output of the earth’s evolution. The latter seems to us the more comprehensive view.

(1) =The material effects of the mental element.=—Lyell long since urged that the direct work of man in changing the face of the earth was slight compared with that of the contemporaneous inorganic agencies. He called attention to the relative insignificance of the quarries, pits, cellars, and other excavations of man, compared with the work of streams, waves, and other inorganic agencies. There is justness in this view, but it needs qualification. It is to be observed that the mental era has but just begun, and that its effects are increasing with a rapidity quite phenomenal when measured by the slow pace of most geologic events. The excavations and transportations of material to-day show an enormous advance on those of Lyell’s day, which was, geologically speaking, but a moment ago. The mile-tons of industrial freightage in the Mississippi basin are to-day not wholly incomparable with the drainage transportation of the same area a century ago. A century ago is named, because the surface was then covered with natural vegetation, and the normal effect of surface erosion, independent of man, was then experienced. At present the indirect effects of man’s action are mingled with those of natural processes, and these indirect effects are probably much more important than the direct ones. The removal of the native vegetation and the cultivation of the soil expose the surface to wash to a degree far beyond that prevalent when the surface was prairie sod, or leaf-carpeted forest, and denudation and transportation have been greatly multiplied in consequence. Not only has this cultivation increased the exposure to erosion, but, by increasing the rate of run-off, it has added to the erosive power of the streams. The ditching of swamps and other tracts of retarded drainage has contributed to this acceleration. The naked, soil-less uplands of some of the once populous kingdoms of the Orient, notably portions of Syria and Greece, are sad witnesses of the accelerated erosion that attends cultivation. The erosion of certain southern fields of the United States in the last forty years is another striking illustration. It is doubtful whether some parts of this region suffered as much erosion in the preceding five centuries as they have during the last one. On the other hand, some compensation is found in the reservoirs established for water-power, and in artificial devices for retarding and steadying stream flow.

In the light of considerations such as these, man may well be regarded not only as a potent geological agent, but as dangerously so to himself. The hope is that the intelligence that has wrought a change of surface conditions serviceable for the present, but dangerous to the future, will be so enlarged as to inspire a still more intelligent control of surface conditions which shall compass the future welfare as well as transient benefit.

=Human modification of the animal and vegetal kingdoms.=—Man’s agency is also coming to be felt powerfully in the modification of the plant and animal life of the land and even to some extent of the sea. The larger animals that are not propagated by man are fast approaching extinction. At the present rate of extension of man’s dominion, a century or so will see the disappearance of nearly every large mammal and reptile that he does not choose to protect or propagate. By way of compensation, certain selected animals are increasing and will doubtless continue to increase. The result is, therefore, likely to be a peculiar assemblage of animal life dependent strictly on the choice of a dominant type, a state of things that has apparently never occurred in an equal degree in the past history of the earth. How far the minor forms of life, especially the insect life, and the denizens of the sea, may be brought under this monopolistic control may not be predicted so easily.

A similar profound transition in vegetation is being forced by man. The native vegetation is rapidly being replaced by selected varieties, and by varieties that take advantage of conditions furnished by man. As the agricultural control of the earth becomes more complete and effective, a result toward which very rapid progress is being made, a new flora of man’s selection will very generally prevail over the whole land surface of the globe. It is doubtful whether at any time in the history of the earth changes of flora and of fauna, and of surface, have been more rapid than those that are now taking place under the accelerating influence of man’s action, and this accelerating influence springs not mainly from automatic or instinctive reaction, but from conscious impulse and intelligent direction.

(2) =The psychological factors as such.=—Are the introduction and the evolution of the psychological factors themselves to be regarded as subjects of geological study? We shall find that, at the outset, the geologic record is a complete blank so far as clear evidence of terrestrial organisms actuated by their own intelligence is concerned; that later, organisms with some apparent consciousness and intelligence appeared, and that the mental element increased apace unto its present attainment. We know that relationships of a sociological nature arose in apparent feebleness, and gradually evolved into more definite, higher, and more complex forms. By sociological factors we mean merely those conscious relations which one organism bears to another, of which the parental and the gregarious impulses are two fundamental expressions. For manifest reasons, the introduction and evolution of the psychological and sociological factors themselves have received little direct recognition as a portion of geological studies. The record of such factors in the fossils of past ages is necessarily obscure and imperfect, and the interpretation of what there is lacks certainty and precision. None the less, this psychological record, with all its imperfections, is beyond valuation, and must, we think, come to be an indispensable factor in the study of psychological and sociological evolution, for it shows, what nothing else can show equally well, the extremely prolonged history of that evolution, and it gives hints of modes and means which no study of existing stages can equally reveal. The organization of the Cambrian trilobites, for example, implies no small development of the senses and of the coordinating faculties even at that early stage, and a study of the relations of these to their fellow creatures opens up the first known chapter in the sociological record of the earth’s inhabitants. From this stage onward the progress in the development of the higher faculties, and of the sociological relations of the leading forms, is one of the most instructive phases of the great history. Such a study reveals the fact that many questions, narrowly supposed to be purely human, have had their prototypes in the earlier experiences of the animal kingdom. Some of these questions have found solutions, temporary or permanent, which passed under the test of ages to whose length human experience affords no parallel, and have received the sanction or disapproval of such tests according as they were well or ill adapted to the actual conditions involved. If one seeks the lessons of history in the largest sense, he cannot wisely neglect the prolonged record of the great biological family.

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.+

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.

REFERENCE TABLE OF THE PRINCIPAL GROUPS OF PLANTS.

{ { Cyanophyceæ, blue-green algæ. { { Chlorophyceæ, green algæ. { { Rhodophyceæ, red algæ. { Algæ and { Phæophyceæ, brown algæ. { algoid forms { Diatomaceæ, diatoms. { { Coccospheres } { { Rhabdospheres } Pelagic algæ(?). +Thallophytes+ { { Charophyta, stoneworts. (Thallus { plants) { { Phycomycetes, algæ-fungi, water-molds. { { Ascomycetes, ascus-fungi, mildews. { Fungi and { Basidiomycetes, basidium-fungi, mushrooms. { fungoid { Æcidiomycetes, æcidium-fungi, “rusts.” { forms { Schizomycetes, “fission-fungi,” bacteria. { { Myxomycetes, “animal fungi,” slime-molds. { { Lichens Symbiont algæ and fungi.

+Bryophytes+ { Hepaticæ, liverworts. (Moss plants) { Musci, mosses.

{Filicales { Filices, true ferns. { { Cycadofilices, cycad-ferns. { {Equisetales { Equisetæ, scouring-rushes, horsetails. +Pteridophytes+ { { Calamites. (Fern plants) {Sphenophyllales. { { Lycopodiaceæ, club-mosses. {Lycopodiales { Lepidodendra. { { Sigillaria and stigmaria.

{ { Cordaiteæ, cordaites. { { Cycadales { Bennettiteæ. { Gymnospermæ { (cycads) { Cycadaceæ. { (Naked seed) { Coniferæ, evergreens. +Spermatophytes+{ { Ginkgoaceæ, ginkgo. (Seed plants) { { Angiospermæ { Monocotyledoneæ, cereals, grasses, etc. { (Covered Seed) { (one-leafed seed). { (Flowering { Dicotyledoneæ, oaks, poplars, peas, etc. { plants) { (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. 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. 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, 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 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, 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, 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.+

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.

REFERENCE TABLE OF THE PRINCIPAL GROUPS OF ANIMALS.

+Protozoa+ { Rhizopoda { Foraminifera. (The simplest { { Radiolaria. animals) { { Flagellata } { Infusoria } Unknown in fossil state. { Gregarina }

+Cœlenterata+ { Porifera Spongiæ { Calcareous sponges. (Sponges, { { Silicious sponges. corals, { Cnidaria { Anthozoa, coral polyps. jellyfishes) { { Hydrozoa, hydroids and medusæ.

+Echinodermata+ { Pelmatozoa { Cystoidea, cystids. (Crinoids, { { Crinoidea, stone lilies. starfishes, { { Blastoidea, blastids. sea-urchins) { { Asterozoa { Ophiuroidea, brittle-stars { { Asteroidea, starfishes. { { Echinozoa { Echinoidea, sea-urchins. { { Holothuroidea, sea-cucumbers.

+Vermes+ { Platyhelminthes } (Worms) { Rotifera } { Nemathelminthes } Rare as fossils. { Gephyrea } { { Annelida, sea-worms.

+Molluscoidea+ { Bryozoa, sea-mosses. (Mollusc-like forms) { Brachiopoda, lamp-shells.

+Mollusca+ { Pelecypoda, lamellibranchs, bivalves. (Molluscs) { Scaphopoda, tusk-shells. { Amphineura, chiton. { Gastropoda, univalves, snails, etc. { Cephalopoda, nautilus, cuttlefish.

+Arthropoda+ { Branchiata { Crustacea. (The { { Trilobita, trilobites. articulates) { { Gigantostraca, horse-shoe crabs. { { Entomostraca, ostracoids, barnacles. { { Malacostraca, lobsters, crabs. { { Tracheata { Myriapoda, centipedes. { Arachnoidea, spiders, scorpions. { Insecta, insects.

{ Cyclostomata, lampreys. { { Pisces { Selachii, sharks. { (fishes) { Holocephali, spook-fishes. { { Dipnoi, lung-fishes. { { Teleostomi, ganoids and teleosus. { { (common fishes). +Vertebrata+ { { Amphibia, amphibians, batrachians. { Reptilia, reptiles. { Aves, birds. { Mammalia { Prototheria, monotremes. { (mammals){ 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.

III. THE ASSOCIATIONS AND ECOLOGICAL RELATIONS OF LIFE.

A. +The Basis of Floras and Faunas.+

Geologic interest is not confined to the kinds of plants and animals that have lived and the contributions they have made to the deposits, but embraces also their assemblage into floras and faunas, and the relations of these assemblages to the prevailing physiographic features. These assemblages and relationships are among the most suggestive factors of the earth’s evolution, and are the most instructive for purposes of comparison with human history, and for forecasting the future of man and of the whole biological kingdom. Moreover, floras and faunas, as such, are used in the correlation of formations, and in this application they give surer results than correlations by individual species. A particular species may live far beyond the usual period of a species, and if fossilized in one region in its early history and in another in its late history, the two formations might be referred erroneously to the same stage. This is far less likely to happen with a whole assemblage of forms. There is a similar liability to error in interpreting migrations on the basis of a single or a few species, for a single species or a few species may be transported by unusual or accidental means, so to speak, when there is no normal pathway for general migration, and when no systematic migration takes place. In most of the great questions that arise concerning the connections and disseverances of the continents, and concerning the unions and separations of the oceans, which are the fundamental causes of the migrations and of the isolations of plants and animals, typical floras and faunas are to be studied, rather than isolated species or sporadic forms. A brief sketch of the leading causes and consequences of these special assemblages of plants and animals may aid in appreciating the underlying significance of floras and faunas, and in interpreting their meaning as they are met in the study of the strata. A part of these grow out of the relations of the organisms to one another, and a part out of the relations of the organisms to their environment.

(1) Assemblages Influenced by the Mutual Relations of Organisms.

(a) =Food relations.=—The relations of food-supply are among the most obvious reasons for assemblages. As animals are dependent directly or indirectly on plants for their food, they must gather where the plants grow, or in the currents in which the plant products are borne. Whatever determines an assemblage of plants also causes, or at least invites, an assemblage of animals. Whatever causes an assemblage of particular plants, invites an assemblage of the particular animals that use these plants. Animals that feed on plants are in turn preyed upon by other animals, and these in turn by others. A whole train of organisms may, therefore, be gathered into a region by the conditions that foster a certain kind of vegetation there. In interpreting the physical significance of such a train, it is obvious that the head of the train carries the fundamental meaning. The dependent creatures that follow the primary forms may be only incidentally, and perhaps very slightly, adapted to the physical environment.

(b) =Adaptive relations.=—Organisms depending on other organisms for food or other necessary conditions of life, present many forms of adaptation the better to secure their food and to use it. These adaptations are the consequences and the signs of the assemblage, and are of the greatest service in interpreting the place and significance of the organisms in the assemblage. Teeth usually reveal the food of their possessors, and hence teeth are among the most significant of fossils. Fortunately their functions require them to be hard and durable, and hence well suited to fossilization. The growth of low plants into trees forced a notable series of adaptations in the animals that fed upon them in the matter of height, of reaching members, of climbing, and probably at length of parachuting and flying. In these and similar ways the floras and faunas took on special phases because of the mutual relations of their members.

(c) =Competitive relations.=—The assembling of plants and animals, with their prodigious possibilities of multiplication, brought competition, and with it a struggle for food which often became a struggle for existence, and out of this grew innumerable modifications of form and habit. These have become so familiar since the great awakening caused by the doctrines of Darwin and Wallace that they need no elaboration here.

(d) =Offensive and defensive relations.=—Within limits, plants are benefited by the feeding of animals and respond by developing seeds and fruits that especially invite such action, their compensation being found in planting and distribution. It is obvious that, on the whole, the continued growth of plants is largely dependent on the renewal of a supply of carbon dioxide through the agency of animals and some plants, bacteria in particular. Otherwise the supply would become so reduced as to greatly limit plant life. It has been estimated that the whole of the present supply of carbon dioxide would be consumed by plants in one hundred years if the consumption continued at the present rate and no carbon dioxide was returned. It is now well known that the so-called decay by which carbon dioxide is freed is due more to microscopic organisms than to inorganic processes. It seems clear, therefore, that the continued activity of plants is largely due to their consumption by animals and other plants. But still, though the larger good of plants is conserved by the predaceous action of animals, and of certain parasitic and saprophytic plants, their individual preservation is often conserved by defensive devices, such as thorns, poisons, bitter compounds, etc. This is notably true in desert regions where the conditions are hard and the total extinction of plants would be threatened if animals were permitted to feed freely upon them. Within the animal world, the preying of one form upon another is the main source of that great struggle for existence which has characterized the whole known history of life, and has been one of the influential factors in shaping the evolution of life and in modifying the special aspects assumed by the floras and faunas of each period.

=Implied forms of life.=—The full meaning of the fossils of any period can only be gathered by duly considering these relationships in their interpretation. The existence of animals implies the existence of plants in supporting abundance, whether the record contains their relics or not; an animal with a protective covering implies an enemy; a tooth of a specific kind implies the appropriate class of food, etc. While inferences of this kind are subject to error, they are at present the only means by which the faunas and floras of most ages can be rounded out into a rational assemblage of organisms, that is, an assemblage that affords the necessary food for its members and an adequate function for the offensive and defensive devices which its members present. Only a small part of the life that lived was fossilized, and only a small part of the fossils actually carried in the strata have been collected, because only a small part of the strata are exposed at the surface. The direct record now accessible is, therefore, very incomplete and hence the need—and in the need the excuse—for adding the forms that are implied by the character of the known fossils.

(2) Assemblages Influenced by Environment.

It has been noted that some animals depend for existence on other animals; that ultimately all animals depend on plants, and that green plants alone can make food directly from inorganic material. Green plants, therefore, head the train of dependencies, and their relations to the physical conditions that surround them are the primal relations.

=Plant societies.=—The control of physical conditions has been sufficient to develop special associations or societies of plants by fostering those adapted to these conditions and eliminating those that are not. Among these are (1) the hydrophytes (“water plants”), embracing those that grow in water or in very wet situations; (2) xerophytes (“drought plants”), embracing the opposite class, which are adapted to very dry situations; (3) mesophytes, including those suited to conditions lying between these extremes, the great middle class to which the prevailing upland vegetation belongs; and (4) the halophytes (“salt plants”), which are dependent on the presence of certain salts, and embrace such plants as are found on the seacoast, around salt springs, on alkaline flats, etc. The characters which distinguish the xerophytes from the hydrophytes and mesophytes have special geological interest, as they aid in determining the climatic conditions, a feature whose interest increases as the variability of the ancient climates is more fully recognized.

Within these greater groups there are special minor associations determined by soil, temperature, topography, subjacent strata, and by the relations of the plants to one another. These natural groups are valuable indications of the agricultural capabilities of the districts occupied by them. They may be regarded as the outcome of Nature’s experiments in crop-raising, running consecutively through thousands of years. They are natural correlations of compatible members into communities of plants. Some members of the society are obviously dependent on others, as certain forms of undergrowth on the shadowing of the upper growth, as of vines upon supporting-trees, etc. There is probably a more occult relation in some cases, the effects of certain plants on the soil being sometimes advantageous to other plants, and sometimes harmful, as illustrated in the conditions that require a rotation of crops.

The chief point of geologic interest lies in the fact that floras are not mere miscellaneous mixtures of plants that happen to live in a given area at a given period, but are organized communities, in a more or less definite sense. They therefore imply more or less definitely the physical conditions which are congenial to them, and thus furnish the basis for interpreting such conditions in the past, so far as the floras are well preserved. The faunas, especially the land faunas, being primarily dependent on the floras, furnish a basis for interpretations of like import.

B. +The Influence of Geographic Conditions on the Evolution of Floras and Faunas.+

The geographic features of the earth impose on organisms a complex series of influences which modify the evolution of life and produce faunal and floral variation on a large scale. The larger assemblages of life, which inhabit a continent or dwell in a great sea, are designated faunas and floras, as well as the smaller assemblages just discussed, but obviously in a broader and in a different sense. The disseverance of the land by the sea, or of the sea by the land, isolates the life and forces independent development. The introduction of cold zones, desert tracts, or other potent climatic belts has somewhat the same effect. So, measurably, does the raising of a mountain range or a plateau, or the sinking of critical portions of the sea-bottom.

=The development of provincial and cosmopolitan faunas.=—If a region is isolated from other regions by the cutting off of all ready means of intermigration, as by the formation of an island from what had been a peninsula, or of an inland sea from what had been a bay, the flora and fauna are developed by themselves without much influx of other forms, and hence become local or provincial. This is usually more marked in the case of the fauna than of the flora, because the latter has more ample means of dispersion, on the whole, and so the fauna may for convenience be taken as the type. A good illustration is the native fauna of Australia which was once connected with Asia, but has long been separated from it. Previous to importations by man, this continent had a very peculiar and distinct fauna, descended from its Mesozoic inhabitants. Most of the isolated islands have peculiar faunas, but in many cases they were isolated from the beginning, having been built up by volcanic action from the bottom of the sea, and their faunas are due to the accidents of transportation and to the development of these sporadic forms in isolation.

It is evident that whenever any geographic change introduces a barrier to migration, the faunas of the dissevered portions will, in all probability, develop along different lines, and will diverge into provincial faunas. On the other hand, any geographic change that unites areas and leads to intermigration, tends to a community of fauna or to cosmopolitanism. These tendencies have been markedly felt all through the geologic ages, and constitute one of the most vital features of their history. When continents are connected, their faunas intermingle and the exchange gives rise to common forms. They tend to blend into one great fauna except so far as the local differences develop those minor assemblages previously discussed. When continents are separated, they tend to develop peculiar faunas, as do islands, but on a larger scale. This is very obvious in the case of the land life, but needs more special statement for the oceans.

The oceans constitute a single body of water with ample connections and stirred by a system of constant circulation. Probably this has been true for most of known geologic time. A single cosmopolitan fauna of the largest type might be expected. This is in a measure realized in the pelagic fauna of the open ocean, though this is somewhat modified by the climatic zones. But the marine faunas that are fossilized in the known strata, and have most geologic interest, are, with rare exceptions, not those of the open ocean, but those of the shore zones and of the shallow seas. Now, although these shore belts and shallow seas are broadly connected with the great ocean body, and are usually regarded as a part of it, they are singularly separated from it, or rather they are singularly separated by it, so far as the life dependent on shallow-water conditions is concerned. To this life, the deep sea is a barrier not quite as effective as the land, but still a barrier. The key to this important fact may be found in a consideration of the vertical distribution of life.

The great horizon of life is at or near the contact zone of the atmosphere with the hydrosphere and lithosphere. Life declines with increasing altitude, partly because of the lowering temperature, and partly because of the increasing tenuity of the atmosphere. The successive changes of plant and animal life with the ascent of mountains and plateaus is familiar. Life declines in descent into the sea chiefly from lack of light, and secondarily from the lowering of temperature. Light is essential to the formation of chlorophyll and, through it, of all other organic compounds. The chlorophyll-forming plants are, therefore, limited to such depths as are penetrated by the rays necessary for the photosynthesis of organic matter. Vision is cut off within 200 to 300 feet, and most plant growth takes place above that depth. Photographic effects become feeble or inappreciable at 1000 to 1200 feet. The photosynthesis of plants is chiefly aided by the lower and middle part of the spectrum, while the ordinary photographic work is chiefly done by the upper end, so that the photographic limit is below the photosynthetic limit. Microscopic plants are sometimes found lower than these limits, but they may have been carried below their working limits by currents or other incidental agencies. For all general purposes, the limiting depth of living carbon-compounding plants may be set at 100 fathoms, as a generous figure—about the average depth of the border of the continental shelf—while the vast majority flourish only in the upper third of this depth.

Life does not cease here, for the products of this surface-life sink to greater depths and are fed upon by forms of sea animals that have become adapted to the dark and cold abyss of the ocean. Obviously, these deep-sea forms are a very distinct type of life, and constitute a fauna of the most pronounced kind, the abysmal fauna. Another distinct fauna occupies the open-ocean surface, the pelagic fauna. Still a third fauna occupies the shallow-water tract, whose bottom lies within the light zone—the photobathic zone—and embraces the animals that are dependent on the plants of this zone, or on its light and warmth, and that are more or less fixed to the bottom or confined to the zone because their food is there.

The physical plane of demarkation between the surface or pelagic fauna and the abysmal fauna is much more distinct and more fundamental than any that is found in ascending above the surface of the sea. The habitat of the shallow-water fauna is limited below by the darkness, limited above by the water-surface, limited at one side by the land, and limited on the other side by the deep sea. It is hemmed in vertically between two planes only a few hundred feet apart. Laterally, it is confined to a narrow belt about the borders of the continents and to the more or less land-girt epicontinental seas. Its vertical limits are fixed, but its lateral extent varies with the relations of the sea to the surface of the continental platforms.

This variation profoundly affects the development of the fauna. When a major deformation of the earth takes place which increases the capacity of the oceanic basins, the water is drawn down into them more fully, and correspondingly retreats from the continental shelf. The shore is thus carried out toward or to the border of the shelf, or even perhaps down to some line on the abysmal slope. In either case, the zone of shallow water suited to the photobathic life is narrowed, and at points it may be practically cut in two. There are, however, shelves and tracts that were below the light zone before, which now are brought within it by the lowering of the sea-level. Into these, as into harbors of refuge, the life migrates so far as it may. But these tracts are less prevalent and continuous than the typical continental shelf, and under the conditions supposed they would be but imperfectly connected with each other by available shallow-water tracts. (The steep shelving shore tracts, although furnishing a shallow-water connection possibly available for some species, would be unsuited to others and, under certain conditions of the sea-currents, would be an effective barrier.) To these limited tracts, therefore, the life of the photobathic type is restricted and measurably isolated, and develops into local and provincial faunas.

After a deforming movement has ceased, the seashore habitually advances, developing a new continental shelf, and in time new epicontinental gulfs and seas. In this it is assisted by the erosion of the continent and the filling of the sea, and probably by the slow settling of the continents. As the sea-shelf broadens, the isolated tracts, the harbors of refuge, become connected, and migration is facilitated. When the connection becomes general and broad, and when epicontinental seas have formed available tracts across the face of the continents, a general commingling of faunas follows, and a cosmopolitan fauna results.

In the same way, but more obviously, when the land is extended and connection between the continents becomes general, there is migration and commingling of the land faunas and floras, and cosmopolitan communities are the result.

It is obvious that the development on the land is the reciprocal of that in the sea. When the seas are extended and their life is tending toward cosmopolitanism, the lands are dissevered, and their life is tending toward provincialism, and vice versa. When, however, the land is greatly extended, it is usually accentuated by mountain ranges, and other products of the deformation which extended it, and these form barriers. Desert wastes and other inhospitable tracts, and even glaciation, are liable to develop as secondary consequences, and to interpose barriers, and hence the cosmopolitanism of the land-life is liable to be less complete than that of the sea-life.

=Restrictive and expansional evolution.=—It is obvious from the last discussion that if the picture of the earth’s movement above drawn be true, the areas available for particular classes of life may vary greatly from age to age. At times the shallow-water sea-life may be forced to retreat into a very narrow tract on the border of the land, and into chance expansions here and there. In being crowded into this limited tract, perhaps also less adapted for a habitat on account of the change, the life is subjected to severe competition and to hard conditions, and must experience in an intensified degree the effects of the struggle for existence. Whatever of evolutionary potency there may be in such a struggle under such restrictive conditions should be revealed in the modifications of the fauna that ensued.

On the other hand, when the shallow seas are generally extending themselves upon the land and the land is being base-leveled, and thus adapted to shallow submergence, the shallow-water life enjoys an enlarging realm, and should reveal the effects of evolution under expansional conditions. In affording a comparison between these opposite and alternating phases of restrictional and expansional evolution, geology makes one of its great contributions to the external causes and conditions of organic evolution. These will come under repeated consideration in the historical chapters.

INDEX.