Page:Encyclopædia Britannica, Ninth Edition, v. 10.djvu/337

Rh USE or ross1Ls.] would preserve one suite of organisms in England, but a very different group at the foot of the Himalaya Mountains, yet the deposits at the two places might be absolutely coeval, even as to months and days. Hence it becomes apparent that while strict contemporaneity cannot be predicated of deposits containing the same organic remains, it may actu- ally be true of deposits in which they are quite distinct. If, then, at the present time, community of organic forms obtains only in districts, regions, or provinces, it In. y have been more or less limited also in past time. Similarity or identity of fossils among formations geographically far apart, instead of proving contemporancity, ought rather to be looked upon as indicative of great discrepancies in the relative epochs of deposit. For in any theory of the origin of species, the spread of any one species, still more of any group of species to a vast dist.anee from the original centre of dispersion, must in most cases have been inconceivably slow. It must have occupied so prolonged in time as to allow of almost indeﬁnite changes i11 physical geography. A species may have disappeared from its primeval birthplace while it continued to ﬂourish in one or more directions in its outward circle of advance. The date of the first appearance and ﬁnal extinction of that species would thus ditfcr widely according to the locality at which we might examine its remains. The grand march of life, in its progress from lower to higher forms, l1as unquestionably been broadly alike in all quarters of the globe. But nothing seems more certain than that its rate of advance has 11ot everywhere been the same. It l1as moved unequally over the same region. A certain stage of progress may have been reached in one quarter of the globe thousands of years before it was reached in another; though the same general succession of organic forms might be fomid i11 each region. The geological formations form the records of these ages of organic development. In every country where they are fully dis-pl-iyed, and where they have been properly exa- min ,-d, they can be separated out from each other according to their organic contents. Their relative age within a limited geographical area can be demonstrated by the mere law of superposition. When, however, the formations of distant countries are compared, all that we can safely aflirm regarding them is that those containing the same or a repre- sentative assemblage of organic remains belong to the same epoch in the history of biological progress i11 each area. They are /u)22z0ta._ria.d ; but we cannot assert that they are contemporaneous, unless we are prepared to include within that term a vague period of perhaps thousands of years. Doctrine of C’ol0nz'es.—M. Barrande, the distinguished author of the Systémc Silurien. dc la Bo/Léme, drew attention more than a quarter of a century ago to certain remarkable intercalations of fossils in the series of Silurian strata of B-_»l1emia. He showed that, while these strata presented a normal succession of organic remains, there were neverthe- less exceptional bands, which, containing the fossils of a higher zone, were yet included on different horizons among inferior portions of the series. He termed these precursory bands “colonies,” and deﬁned the phenomena as consisting in the partial co-existence of two general faunas, which, con- sidered as a whole, were nevertheless successive. He sup- posed that during the later stages of his second Silurian fauna in Bohemia the first phases of the third fauna had already appeared, and attained some degree of development in some neighbouring but yet unknown region. At inter-- vals, corresponding doubtless to geographical changes, such as movements of subsidence or elevation, volcanic eruptions, &c., communication was opened between that outer region and the basin of Bohemia. During these intervals a greater or less number of immigrants succeeded in making their way into the Bohemian area, but as the conditions for their GEOLOGY 323 prolonged continuance there were not yet favourable, they soon died out, and the normal fauna of the region resumed its occupa11cy. The deposits formed during these partial interruptions, notably graptolitic sehists, accompanied by igneous sheets, contain, besides the invading species, remains of some of the indigenous forms. Eventually, however, on the ﬁnal extinction of the second fauna, and, we may sup- pose, on the ultimate demolition of the physical barriers hitherto only occasionally and temporarily broken, the third fauna, which had already sent successive colonies i11to the Bohemian area, now swarmed into it, and peopled it tiil the close of the Silurian period. This original and ingenious doctrine has met with much opposition on the part of geologists and palzeontologists. Of the facts cited by M. Barraude there has been no ques- tion, but other explanations have been suggested for them. It has been said, for example, that the so-called colonies are merely bands of the Upper Silurian rocks or third fauna, which by great plications have been so folded with the older rocks as to seem regularly interstratiﬁed with them. But the author of the S3/stéme ;S'ilzu-ien very justly contends that of such foldings there is no evidence, but that, on the contrary, the sequence of the strata appears normal and undisturbed. Again it has been urged that the difference of organic contents in these so-called colonies is due merely to a difference in the conditions of water and sea- bottom, particular species appearing with the conditions favourable to their spread, and disappearing when these ceased. But this contention is really included in M. Barrande’s theory. The species which disappear aml re- appear in later stages must l1ave existed in the meanwhile outside of the area of deposit, which is precisely what he has sought to establish. Much of the opposition which his views have encountered has probably arisen from the feeling that if they are admitted they must weaken the value of pal-teontological evidence in deﬁning geological horizons. A p:1laeontologist, who has been accustomed to deal with certain fossils as unfailing indications of particular portions of the geological series, is naturally unwilling to see his generalizations upset by an attempt to show that the fossils may occur on a far earlier horizon. If, however, we view this question from the broad nat- ural history platform from which it was regarded by M. Barrande, it is impossible not to admit that such phenomena as he has sought to establish in Bohemia nmst have con- stantly occurred in all geological periods and in all parts of the world. N 0 one now believes in the sudden extinction and creation of entire falfnas. Every great fauna in the earth’s history must have gradually grown out of some pre- existing one, and must have insensibly graduated i11to that which succeeded. The- occurrence of two very distinct faunas in two closely consecutive series of strata does not prove that the one abruptly died out and the other suddenly appeared in its place. It only shows, as Darwin has so well enforced, the imperfection of the geological record. I11 the interval between the formation of two such contrasted groups of rocks the fauna of the lower strata must have continued to exist elsewhere, and gradually to change into the newer faeies which appeared when sedimentation recommenced with the upper strata. Distinct zoological provinces have no doubt been separated by narrow barriers in former geo- logical periods, as they still are to-day. There seems, therefore, every probability that such migrations as M. Barrande has supposed in the case of the Silurian fauna of Bohemia have again and again taken place. Two notable examples will be given in later pages, one in the Lower and one in the Upper Old Red Sandstone of Scotland. Gaps in the Geological It’ecord.—The history of life has been very imperfectly preserved in the stratiﬁed parts of the earth’s crust. Apart from the fact that, even under the