Page:The American Cyclopædia (1879) Volume XII.djvu/824

ments, continued perhaps for a considerable time, yet on a recurrence of the former physical conditions some of the species of the preceding epoch do reappear. So dependent upon and connected with the surrounding physical influences is the fauna, that sometimes, after a considerable interval of change, and the existence of an entirely different fauna, a recurrence of former conditions, though not bringing back the same species, will bring a group of similar forms and of the same genera, and sometimes species scarcely distinguishable from those of the preexisting fauna.—Since all observation has proved that no species continues through all the geological eras, and few if any are found in more than two periods, we are warranted in inferring a law of limited duration in time for each species. The effects of this law may appear, in the general results, in two ways: either that each species is altogether independent of others in its appearance and disappearance, or that those species appearing together for the first time have also disappeared together. In the first case there would be a continued series of modifications in animal populations, and consequently an impossibility of finding in the zoölogical characters any defined limits for geological periods. In the second case there would be strong lines of demarcation between the disappearance of species and the appearance of those which replace them. In order to decide among the geological faunas whether the rule is the independence of the species or the combination of the above two conditions, and taking into consideration only those faunas living in the same or proximate localities, it would be inferred that the phenomena which can produce a renewal of the zoölogical population must extend their action to almost the totality of species. The principal proofs of this conclusion are to be found in the observation of geological facts. Individual species nevertheless do die out without geological changes, but of themselves in the midst of conditions in which they lived and flourished. If in fossiliferous deposits we observe the line of contact of beds, it will be found that the fossil forms are nearly always very restricted. Sometimes a formation characterized by its fossils succeeds without gradation to another not less distinct in its fossil characters. Sometimes a non-fossiliferous bed occurs between two fossiliferous formations; but rarely do we find a deposit containing in their normal condition the fossils of the bed below it, mingled with those of the bed by which it is succeeded. We shall usually find that where there is a mingling of the species of two formations, this single member containing them is the representative of two distinct beds in some other part of the geological field, and combines in some degree the physical characters of both. In some cases this fact may perhaps be explained by admitting that the cause of extinction has operated generally upon the fauna, but that this action has been locally circumscribed, while the two faunas continued their existence in distant localities, beyond the reach of this influence. The question of the duration of species has divided naturalists into two parties. Those who maintain their limited duration hold that the extinction of all contemporaneous faunas took place simultaneously over the whole extent of their geographical distribution; that the species of each geological period have lived only in that period; and that no one species can be found in formations of different ages. They hold that species so occurring which have been considered identical must be in reality distinct. The solution of this question is of very great palæontological interest. If fossils are special to the formations, they characterize them with a complete certainty. If some are special and some are common to several formations, a part only can furnish conclusions, and hence a source of uncertainty and chances of error. The united experience of observers goes to prove that some species are restricted to certain formations, and are therefore characteristic of those formations, while others have a wider geological range, and cannot be wholly relied on for the determination of age among the strata; and in the study of a group of fossils, no careful naturalist will do otherwise than base his conclusions accordingly. It is nevertheless true that sometimes, from erroneous determinations, the same fossil species have been catalogued as from different formations; and there has also been a want of unity of opinion among geologists in regard to the limits of formations, while in disturbed regions it may not have been possible to define the limits, and thus the collections show an admixture of fossils which are elsewhere known to belong to different formations. Established facts in palæontology prove that the limited duration of species is the general rule; but at the same time the rule has exceptions. 1. Some species, being more robust, more prolific, or living under more favorable circumstances, may have resisted causes of destruction which prevailed over their associates. 2. The causes of extinction have rarely operated uniformly or with the same force over the entire geographical extent of the fauna of any period. Examples might be cited where the coming in of 1,000 or 2,000 ft. of sediment destroyed the previous fauna over many hundreds of miles in extent; and yet beyond the thinning margin of this deposit remains of the preceding fauna are found in contact with the overlying formation, and the fossils of the two eras are mingled, although apparently not having lived at the same time. We infer, therefore, that upon the extreme limits of the disturbed areas there may have been basins where some of the animals escaped, and thus afterward mingled with the new population. In general such basins, being beyond the geological movement, are marked by the small accumulation of sediments and