Geology, Vol. 1 [of 3]

by Thomas C. Chamberlin; Rollin D. Salisbury

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[298] 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.[299]—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.[300] 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.[301]—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.[302]

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.[303] 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.