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.