Geology, Vol. 1 [of 3]

by Thomas C. Chamberlin; Rollin D. Salisbury

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.