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.