Page:Encyclopædia Britannica, Ninth Edition, v. 7.djvu/302

284 284 DISTRIBUTION [AXIMAL. The Entoinostraca, Trilobites, and Phyllopods, come before the higher Decapod Crustacea, and of these the highest form the Brachyura appears much the latest. Again, all the aquatic classes of invertebrates appear in abundance before the earliest of the aquatic vertebrates fishes make their appearance. These are followed by Amphibia, and later still by true reptiles. The more highly organized birds and mammals appear later, and almost simultane ously. There are, ib is true, many anomalies, the higher and more complex organisms in some of the minor groups appearing before the lower ; but these cases generally occur in the oldest (Palaeozoic) formations, where, on the principles already laid down, the record must be necessarily more imperfect. In the Mesozoic and Tertiary formations the succession is more regular, and accords better with the grade of organization of the several groups, and the best examples of this are to be found among the Mammalia of the Tertiary period, the series of which is, in some groups, tolerably complete. Thus, among the Ungulata we find in the Eocene deposits the remains of a number of generalized types, such as the Palceotherium, allied to the horse, tapir, and rhinoceros ; Lophiodon, an ancestral form of tapir ; Anoplotherium, intermediate between pigs and ruminants ; Pliolophus, allied to the tapir and horse ; and the North American Orohippus, a remote ancestor of the horse. 1 This last-named animal, Professor Marsh tells us, had four toes in front and three behind, and was no larger than a fox ; yet an almost perfect series can be traced, in succeeding deposits, of animals with smaller and smaller lateral toes, the size and speed increasing, the head and neck becoming longer, the canine teeth decreasing in size, the bones of the fore-arm consolidating, and other modifications successively taking place till we come, by almost imperceptible grada tions, to an animal so completely unlike the one we started from as our existing horse. In like manner we have the extinct families of the Anoplotheridce, Anthracothcridce, Oreodontidce, and many groups of doubtful affinities, which seem to be ancestral forms from which sprung the swine, hippopotami, and all our ruminants. These become more specialized in the Miocene; but it is only in the later Miocene and Pliocene that we find true deer, camels, oxen, and antelopes. So, the oldest form of the Carnivora, found in the very lowest bed of the Eocene formation, is the Arctocyon, one of the generalized types which cannot be referred to any existing family. A little later the CanidcB and Viverridce appear, while the more special ized and highly organized Felidce are not found till the Miocene period. To exhibit in detail the succession and affinities of extinct forms is the province of paleontology ; we can here only give the chief facts in outline, which however are sufficient to render intelligible the great principle which almost all paleontologists have arrived at, viz. that extinct animals exhibit more generalized struc tures, as compared with the more specialized structures of recent animals. (Owen s Palaeontology, p. 406.) Having now laid before our readers a sketch of the more important facts of the distribution of animals in time, we will conclude this branch of our subject with a brief discussion of its bearing on the theory of evolution, and on the imperfection of the geological record. The abrupt ness with which animal remains in considerable variety first appear in very ancient deposits is undoubtedly a most remarkable phenomenon. With the exception of the still somewhat doubtful Eozoon, the vast series of Laurentian rocks have produced no fossils. But the moment we enter 1 A still more remote ancestral form Eohippus has since been discovered in the lowest Eocene deposits of West America. See Huxley s American Addresses, p. 90. the Cambrian formation we at once meet with a somewhat extensive series of complex and varied organisms. Besides the Brachiopoda we have Pteropoda, a by no means low form of Mollusca ; while the Trilobites and Phyllopods exhibit a considerable amount of specialization. AJniost as early, we have sponges, annelids, star-fishes, eucrinites, lamellibranchiates, and Orthoceratidse, a variety of divergent and complex types, which, on any theory of development, indicates a very long successsion of ancestral forms. But we must also bear in mind that the few fossiliferous deposits of this early age cannot possibly have made us acquainted with more than a minute fraction of the organisms which then existed on the whole earth. We are therefore compelled to believe that the absence of all remains of more ancient forms of life in the pre-Cambrian rocks is fallacious, and clue solely to no record of them having been preserved, or, if preserved, to their not having been discovered by us. This conclusion is supported by analogous facts which occur and recur in every succeeding formation. The highly specialized corals and fishes of the Silurian rocks must have had ancestors in Cambrian times of which we know nothing ; and the sudden appearance of perfectly developed winged insects in the Devonian forma tion, plainly tells us that during countless unrecorded ages various lower forms of terrestrial Annulosa must have been gradually developing into these marvellously specialized types, yet these lower forms (Myriapods, &c.) only appear as fossils in the succeeding Carboniferous formation. Such highly organized insects also imply the existence of vegetation, and, by analogy, of other terrestrial animals of an equally high grade of development. Hence the dis covery of these winged insects (which can, with great pro bability, be classed in one of o*ur existing orders the ISTeuroptera) opens up to the imagination of the evolutionist a wonderful picture, far removed from the dreary waste of waters which was once thought to characterize the epoch of the early Palaeozoic formations. Geologists, indeed, have long taught us that the vast piles of sedimentary rocks of the Silurian, Cambrian, and even the Laurentian period necessarily implied the co-existence of extensive continents or island s whose denudation could alone produce them ; and now the theory of evolution enables us to clothe these ancient lands with vegetation and people them with animal life, since it is only thus that we can find space and time sufficient for the development of the wonderful insects, the land shells, the Amphibia, and the reptiles, all of which appear suddenly, in perfect and completely organized forms, in some parts of the Paleozoic series. When we consider that we have indications of the existence during the Carboniferous age of such diversified and highly specialized types of Annulosa as myinapods, spiders, cock roaches, locusts, dragon-flies, ephemeras, lamellicorn-beetles, and bombyciform moths, so that it is highly probable that no fresh ordinal type of insects has originated during all succeeding ages, and when we further consider that all these are specialized modifications of simple Annulosa, we shall be forced to conclude that, whatever time may have elapsed from that epoch to the present day, a far longer time is required, antecedent to the Carboniferous period, to allow of the development of such varied terrestrial forms of life. As bearing upon this question it is important to consider how scattered and fragmentary are the few indications of mammalian life older than those of the Tertiary period. Sir Charles Lyell tells us, that up to the beginning of the present century it was a generally received dogma in ecology that the Mammalia had not been created before the Tertiary period; and the first discovery of the jaw-bone of a small Marsupial in the lower Oolite caused as much sensation as would now be excited by our finding a