But let us leave for the present the somewhat isolated case of Eozoon, and the few scattered forms of the Huronian, and go on farther to the early Cambrian fauna. This is graphically presented to us in the sections in South Wales, as described by Hicks. Here we find a nucleus of ancient rocks, supposed to be Laurentian, though in mineral character more nearly akin to the Huronian, but which have hitherto afforded no trace of fossils. Resting unconformably on these is a series of slates and sandstones, regarded as Lower Cambrian, the Caerfai group of Hicks, and which are the earliest holding organic remains. The lowest bed which contains indications of life is a red shale near the base of the series, which holds a few organic remains. The species are a Lingulella, worm burrows and a Trilobite.[70] Supposing these to be all, it is remarkable that we have no Protozoa or Corals or Echinoderms, and that the types of Brachiopods and Crustaceans are of comparatively modern affinities. Passing upward through 1,000 feet of barren sandstone and shale, we reach a zone in which many Trilobites of at least five genera are found, along with Pteropods, Brachiopods and Sponges. Thus it is that life comes in at the base of the Cambrian in Wales, and it may be regarded as a fair specimen of the facts as they appear in the earlier fossiliferous beds succeeding the Laurentian. Taking the first of these groups of fossils, we may recognise in the worms representatives of those that still haunt our shores, in the Trilobite a Crustacean or Arachnoid of no mean grade. The Lingulellæ, whether we regard them as molluscoids, or, with Professor Morse, as singularly specialized worms, represent a peculiar and distinct type, handed down, through all the vicissitudes of the geological ages, to the present day. Had the Primordial life begun with species altogether inscrutable and unexampled in succeeding ages, this would no doubt have been mysterious; but next to this is the mystery of the oldest forms of life being also among the newest. One great fact shines here with the clearness of noon-day. Whatever the origin of these creatures, they represent families which have endured till now in the struggle for existence without either elevation or degradation. Here, again, we may formulate another creative law. In every great group there are some forms much more capable of long continuance than others. Lingula among the Brachiopods is a marked instance.
[70] Probably of the genus Olenellus.
But when, with Hicks, we surmount the mass of barren beds underlying these remains, which from its unfossiliferous character is probably a somewhat rapid deposit of Arctic mud, like that which in all geological time has constituted the rough filling of our continental formations, and have suddenly sprung upon us many genera of Trilobites, including the fewest-jointed and most many-jointed, the smallest and the largest of their race, our astonishment must increase, till we recognise the fact that we are now in the presence of another great law of creation, which provides that every new type shall be rapidly extended to the extreme limits of its power of adaptation.
That this is not merely local is evidenced by the researches of Matthew and Walcott in the oldest Cambrian of America, where a similar succession occurs, but with this difference, that in the wider area presented by the American continent we find a greater variety of forms of life. Walcott records up to 1892 no less than 67 genera and 165 species in the oldest Cambrian of America. These include representatives of the Sponges, Hydroids, Corals, Echinoderms, Worms, Brachiopods, Bivalve and Univalve Mollusks and Crustaceans, or in other words, all the leading groups of invertebrate animals that we find in the sea at present. Of these the dominant group is the Crustaceans, including Trilobites, numbering one-third of the whole; and these with the univalve Mollusks and the Brachiopods constitute the majority, the other groups having comparatively few species. What a marvellous incoming of life is here! Walcott may well say that on the theory of gradual development we must suppose that life existed at a period far before the Cambrian—as far, indeed, as the Cambrian is before our own time. But this would mean that we know only half of the history of life; and perhaps it is more reasonable to suppose that when the conditions became favourable, it came in with a rush.
Before considering the other laws that may be inferred from these facts, however, let us in imagination transfer ourselves back to the Primordial age, and suppose that we have in our hands a living specimen of one of the larger Trilobites, recently taken from the sea, flapping vigorously its great tail, and full of life and energy; an animal larger and heavier than the modern king-crab of our shores, furnished with all the complexity of external parts for which the crustaceans are so remarkable, and no doubt with instincts and feelings and modes of action as pronounced as those of its modern allies, and, if Woodward's views are correct, on a higher plane of rank than the king-crab itself, inasmuch as it is a composite type connecting Limuli with Isopods, and even with scorpions. We have obviously here, in the appearance of this great Crustacean or Arachnoid, a repetition of the facts which we met with in Eozoon; but how vast the interval between them in geological time, and in zoological rank! Standing in the presence of this testimony, I think it is only right to say that we possess no causal solution of the appearance of these early forms of life; but in tracing them and their successors upward through the succeeding ages, we may hope at least to reach some expressions of the laws of their succession, in possession of which we may return to attack the mystery of their origin.
First, it must strike every observer that there is a great sameness of plan throughout the whole history of marine invertebrate life. If we turn over the pages of an illustrated text-book of geology, or examine the cases or drawers of a collection of fossils, we shall find extending through every succeeding formation, representative forms of Crustaceans, Mollusks, Corals, etc., in such a manner as to indicate that in each successive period there has been a reproduction of the same type with modifications; and if the series is not continuous, this appears to be due rather to abrupt physical changes; since sometimes, where two formations pass into each other, we find a gradual change in the fossils by the dropping out and introduction of species one by one. Thus, in the whole of the great Palæozoic Period, both in its Fauna and Flora, we have a continuity and similarity of a most marked character.
It is evident that there is presented to us in this similarity of the forms of successive faunas and floras, a phenomenon which deserves very careful sifting as to the question of identity or diversity of species. The data for its comprehension must be obtained by careful study of the series of closely allied forms occurring in successive formations, and the great and undisturbed areas of the older rocks in America seem to give special facilities for this, which should be worked, not in the direction of constituting new species for every slightly divergent form, but in striving to group these forms into large specific types.[71]
[71] The Rynchonellæ of the type of R. plena, the Orthids, of the type of O. testudinaria, the Strophomenæ of the types of S. alternata and S. Rhomboidalis, the Atrypæ of the type of A. reticularis, furnish cases in point among the Brachiopods.
There is nothing to preclude the supposition that some of the groups mentioned in the note are really specific types, with numerous race modifications. My own provisional conclusion, based on the study of Palæozoic plants, is that the general law will be found to be the existence of distinct specific types, independent of each other, but liable in geological time to a great many modifications, which have often been regarded as distinct species.[72]
[72] "Geological History of Plants."