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IFkales and Seats ; Mr Andrew Murray's Geographical Distribution of the Mammalia.)

General Relations of Marine icith Terrestrial Zoological Regions.—The general facta of distribution of marine animal a now adduced accord very well with what we know of those terrestrial changes which have led to the actual distribution of land animals. The great In Jo-Pacific region —so well marked in every important group of marine animals—probably owes its individuality to the fact that Australia bas been isolated during the whole of the Tertiary, and probably during much of the Mesozoic epoch, while numerous islands in the Indian and Pacific oceans have always afforded an extensive shore-line favourable to the development of aquatic forms of life. The Atlantic has probably been for long periods even more inclosed than it is now, owing to the greater southward extension of South Africa and South America; while the profound depths of its central channel have served as a barrier between the' inhabitants of the shallow waters of its eastern and western shores. In like manner the great trough of deep water which separates the most eastern groups of the Pacific islands from the west coast of America has necessarily led to the establishment of distinct oceanic faunas in these regions ; while this very fact—the remarkable distinctness of the Pacific and West American faunas—tells us plainly that this barrier of deep ocean is one of the ancient features of the earth's surface.

We shall find, too, that many of the details, and not a few of the anomalies, of the distribution of marine animals become intelligible from our knowledge of past geographical changes. The considerable affinity between the Crustacea, Mollusca, and fishes of the eastern and western coasts of America exactly corresponds to the fact, clearly established by a consideration of the distribution of living and extinct land animals, that these oceans have been united, at several distinct periods, by two or more channels over what is now Central America, the final union of the two continents being comparatively recent The fact that the uniting channels were always situated within the same limited area sufficiently explains the considerable amount of generic and specific difference of two faunas ranging over coast-lines running north and south for many thousand miles on the opposite sides of great continents. The curious fact (only recently established) that so deep and extensive an inland sea as the Mediterranean contains but few peculiar marine animals, becomes quite intelligible when we consider that till middle or late Tertiary times it consisted of two inland seas or lakes. Such inland seas are always very poor in animal life; and it is therefore not surprising that the Mediterranean should now contain hardly any forms but such as it has received from the Atlantic, or from the Red Sea during a submergence of the Isthmus of Suez. The numerous allied or even identical forma in the northern and southern oceans, which are not found in the intervening warm, regions, are more difficult to explain. Mr Darwin believes that such facte are due to the action of the glacial period, which at its height may have cooled certain tracts of the tropical ocean sufficiently to allow temperate forms to cross from the northern to the southern hemisphere or the reverse. Perhaps, however, the agency of icebergs may have been sufficient without any permanent cooling of the equatorial ocean; for even now these huge floating glaciers often reach to 40° N. lat. and 35° S„ and, Captain Maury assures us, sometimes even reach the tropics. We may therefore well suppose that during the height of the glacial period icebergs would not only regularly reach the tropics, but, carried on by currents in definite lines, might often pass across the equatorial zone, carrying with them a girdle of cold water in which many inhabitants of the Arctic or Antarctic seas might safely

make the passage to another hemisphere. The fact that many forms of plants peculiar to cold or temperate regions are found scattered on isolated mountain summits in the tropics is, as Mr Darwin has shown, to be explained only by the influence of an extreme glacial period, and this must have produced analogous effects on the inhabitants of the ocean. {Origin of Species, 6th ed. p. 330.)

Distribution Of Animals In Time.

This subject will necessarily be treated in some detail under the articles Geology and Palaeontology. Here we shall only sketch its outlines and general principles.

The past history of living things as revealed by geology is an ever-changing panorama. At each successive stage some forms disappear, while new ones take their place. The farther we go back the more unlike is the general assemblage of animals and plants to that which now exists. If we confine our attention to any one class or order of animals, we find that it makes its first appearance at some definite epoch, and, under successively changing forms, either continues till the present time, or reaches a maximum, diminishes, and finally disappears. Thus some groups are altogether modern, others extremely ancient; some have run through all their phases in a comparatively short period, others have lived from the earliest epochs of the earth's history of which we have any record and still survive. If we could be sure that the numerous fossils yet discovered gave us anything like an adequate idea of all the varied forms of life that had ever lived upon the globe, and the order in which they had appeared, we should be in a position to decide as to the truth and value of the development hypothesis. But the more we examine the question, the more certain it becomes that the " geological record," as it is termed, is extremely imperfect, and that the whole of the extinct animals which we have discovered do not form any fair representation of the entire series that have lived upon the earth. This is the case even with the more recent deposits and; those which are richest in animal remains; but as we go back into the past the record becomes more and more imperfect, till in the Secondary, and still more in the Palaeozoic formations, we only have preserved to us a few scattered fragments, equivalent perhaps to a few pages with here and there a short chapter taken at random out of a voluminous history. The causes of this necessarily imperfect record of the past have been fully discussed by Sir Charles Lyell and Mr Darwin; we need only refer here to two general causes of such imperfection. The first is, that every aqueous deposit is formed by the wearing down of previous depostg, so that the records of one age are, to a large extent, necessarily destroyed to provide the records of the next, which in its turn is destroyed in a succeeding age. The other cause of imperfection is, that extensive areas are always sinking (to allow new deposits to be formed over them), and are being subjected to subterranean heat to such an extent as to change their texture and obliterate their fossils, when they become crystalline or metamorphic rocks. The more recent deposits so acted on will rarely have had time to have become raised above the sea-level, and subsequently exposed by denudation; yet certain Eocene, strata in the Alps are stated by Sir C. Lyell to be truly metamorphic (Students' Elements of Geology, p. 600). The older a formation is, therefore, the more frequently will it hare been exposed in one area or another to this metamorphic action; and it follows that, going backward in time, we shall at last come to a period, all the formations antecedent to which will have become metamorphosed, and their fossils, if any, obliterated. We appear to have almost reached such a state of things at the base of the Palaeozoic rocks; and there is good reason to believe

VII. — 36

that an extensive series of fossiliferous deposits may have once existed, whose record of the earlier stages of the history of life upon the earth has been either destroyed by denudation or obliterated by internal heat. This being the case, we must carefully distinguish between positive and negative evidence ; aud we may also fairly apply such principles as can be established by means of the fuller record afforded by the Tertiary deposits, to interpret the more scanty and fragmentary record with which we have to deal in the older rocks. We will now proceed to sketch very briefly the successive stages of the development of animal life as indicated by the materials at our command.

The lowest and most ancient of all the stratified rocks is the Lanrentian, consisting of crystalline beds of gneiss, mica-schist, quartzite, and limestone, reaching in Canada the aggregate thickness of 30,000 feet The whole man was long thought to be destitute of organic remains, till in one of the beds of limestone in the lower part of the series a curious structure was discovered, which is held by Dr Carpenter and Professor Rupert Jones, who have made a special study of the Foraminifera, to be the fossilized remains of one of that group of the Protozoa. It has been named Eozoon eanadense, and if really organic (which is denied by some naturalists of eminence) is by far the oldest trace of animal life. The Upper Lanrentian deposits, 10,000 feet,thick, lie nnconformably on the lower, and seem to be entirely destitute of fossils.

The next formation is the Cambrian, largely developed in Wales, Scandinavia, Bohemia, and North America, and consisting of a variety of distinct deposits. But in the very lowest of these (the Longmynd group) abundant organic remains have recently been found, comprising

Serfectly developed brachiopodous and pteropodous [ollusca, entomostracous Crustacea, and Trilobites. In the overlying beds of the same formation similar forms abound, and are accompanied by sponges, annelids, graptolites (which are supposed to be peculiar extinct Hydrozoa), starfishes, and encrinites. Here also first appear lamellibranchiate .Mollusca, belonging to the families Arcadee, Nuculidae, and Atlantidce, and there are even some Orthoceratidce, belonging to the highest order of molluscs—Cephalopoda. The Trilobites are already wonderfully varied, the smallest and largest kinds being found here (one 2 feet long), species with the least and with the greatest number of rings, blind Trilobites, a. d others with the most largely developed eyes. (Lyell's Students' Elements of Geology, pp. 483, 485, 634.)

We next come to the Silurian formation, in which we first meet with corals, of the three great divisions Bugosa, Tabulata, and Perforata,—ostracode Crustaceans, Trilobites in enormous variety, Merostomato—extinct Crustaceans of gigantio size, Echinoidea (Palaxhinus), and true gasteropodous Mollusca. And lastly, in the Upper Silurian deposits, we find vertebrates, whose first representatives are several genera of fishes belonging to the Ganoid and Plogiostomous groups.

In the succeeding Devonian formation we find an abundance of new families of fishes, a fresh-water mussel of the living genus Anodon, and no less than six forms of winged insects. These have been found in the Devonian' rocks of New Brunswick, and are considered by Mr Scudder to be ancient forms of Neuroptera.

The Carboniferous formation is very rich in animal as well as vegetable remains; and, along with most of the animals already met with, we find several higher types of great interest. The higher macrurous Crustacea (Anthrapalcemon) are here first met with, as are true air-breathing molluscs, numerous specimens of the living genera, Pupa and Zonites, having been found in the coal-fields of Nova Scotia. Along with theso are insects of various orders—

Myriapoda, scorpions, spiders, Orthoptera, Neuroptera, Coleoptera, and even Lepidoptera, Here, too, we meet with air-breathing vertebrates—the Labyrinthodonts, ancient forms of Amphibia which occur in considerable abundance and variety. (Lyell's Students' Elements, p. 408; Annates de la Sociile Entomologique de Bdgique, 1875, torn xviiL, where a wing from the coal-measures, closely resembling those of moths belonging to the family Saturniidce, is photographed.)

In the Permian formation, which closes the series of Palaeozoic rocks, we have the important addition of true Locertian reptiles (Protorosaurus), which, according to Professor Huxley, differ wonderfully little from some living groups. What are supposed to be Chelonian footprints have been discovered in the Permian sandstones of Dumfriesshire. (Huxley's American Addresses, p. 41.)

Entering the Secondary period with the Triassic formation, we at once meet with higher forms of life. Among Crustacea we first find traces of the brachyuruus division of Decapods (Etheridge, in Lyell's Students' Elements, p. 632) and many new forms of Mollusca. Among reptiles the Dinosauria, Dicynodontia, Plesiosauria, and Crocodilia appear; what seem to be"undoubted footprints of birds have been found in the New Red Sandstone of Connecticut (see figures in Lyell's Students' Elements, p. 371, and Nicholson's Palaeontology, p. 389) J and all improbability of this early appearance of birds is removed by the fact that a little higher in the same formation remains of a true Mammalian have been undoubtedly discovered. This is the Microlestes, founded on well-preserved teeth from a bone bed in the Upper Trias of Wurtemberg, and since found also in the Rhaetic beds of Somersetshire; while in rocks supposed to be of the same age in North Carolina the lower jaw of an allied form (Dromatherium) has been obtained. Both are believed to be Marsupials, and most nearly allied to the Myrmecobius of Australia.

In the Jurassic or Oolitic period, the main forms of life which have already appeared are further developed. Insects of all orders ore found, and they can mostly be classed in existing families and even genera, as—Locusta, Nepa, Sphinx, Termes, Ephemera, Agrion, jEslma, Agrion, Primus, Libellvla. (Rev. P. B. Brodie, in Proe, Warwick' shire Nat. Hist. Soc, 1873.) Among reptiles, Chelonia and Ichthyosauria are added. Of birds we have the longtailed and feathered Archaeopteryx; while no less than eight genera of small Mammalia have been discovered, most of them Marsupials, though some may have been ancestral forms of Tnsectivoro. Many living genera of shells, both marine and fresh-water, first appear; and among fishes, true sharks of the existing family Notidanidce.

In the Cretaceous period, we make a still further approach to living forms. The highest Crustacea (Bra. chyura) are tolerably abundant, and tlje living genus Cancer appears. Mollusca (Lamellibranchiate and Gasteropoda) are represented by a number of living genera. Malacopterous fishes now appear. Reptiles are still mostly of extmct types — Pterosauria, Ichthyosauria, Dinosauria, 4c.; but among birds we find some allied to existing waders, as well as the curious extinct group of Odontornithes, or toothed birds. (Marsh, in American Jour, of Science and Arts, vol x. 1875.)

When we pass over the great asm of time which separates the Mesozoic from the Cainozoic or Tertiary period, we at once come upon a host of new forms closely resembling those which now live upon the earth. The majority of living genera of Mollusca now appear, with a gradually increasing proportion of living species, as we pass from the Eocene to Miocene and Pliocene times; the highest forms of Crustacea are plentiful; Insects of all orders, and almost all of living genera, abound; fishes of tiring genera gradually appear, and true snakes (Ophidic) are first met with. Among birds, all the existing orders, many families, and some living genera appear in the Miocene period. Mammalia, however, exhibit the most surprising advance. Ancestral forms of all the existing orders are found in the Eocene formation ; in the Miocene, most living families are well developed; while in the Pliocene and post-Pliocene deposits we find the genera and species for the most part closely resembling those that still The Entomostroca, Trilobites, and Phyllopods, come before the higher Decapod Crustacea, and of these the highest form—the Brachynra—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 simultaneously.

inhabit the earth. The following diagrammatic table will enable the reader better to comprehend the main facta which we have here endeavoured to set forth. It comprised only the larger or more important groups of animals, and of each of these the known range in time is indicated by a thick line. It has not been attempted to show the breaks which occur in our knowledge of the range of a group, since no one now doubts that where any type appears in two remote periods it must have beeu in existence during the whole

Table Showing The Range In Time Op The More Important Groups Op Animals.

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There are, it 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 Mesozoio 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 Palaeothcrium, allied to the horse, tapir, and rhinoceros; Lophiodon, an ancestral form of tapir; Anoplotherium, intermediate between pigs and ruminants; Fliolophus, allied to the tapir and horse; and the North American Orohippia, a remote ancestor of the horse.1. This last-named animal, Professor Marsh tells us, had four toes in front and thres 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 gradations, 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 Anoplolheridce, Anthracotheridce, Ortodontidoe, 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 iu 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 Canida and Viverridoe appear, while the more specialized and highly organized Fclidcc are not found till the Miocene period. To exhibit in detail the succession and affinities of extinct forms is the province of palaeontology; we can here only give the chief facts in outline, which however are sufficient to render intelligible the great principle which almost all palaeontologists have arrived at, viz.—that extinct animals exhibit more generalized structures, 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 abruptness 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 Eotoon, the vast series of Laurentian rocks have produced no fossils. But the moment we enter

'A still more remote ancestral form Eohtppui has aiace been discovered in the lowest Eocene deposits of West Aiucnua. See Unxleys American \ddruM4, p. 90.

the Cambrian formation wo at once meet with a buinewhat extensive series of complex and varied organisms. Besides the Brachiopoda we have Pteropoda, a by uo means low form of Molluscs; while the Trilobites and rhyllopods exhibit a considerable amount of specialization. Almost as early, we have sponges, annelids, star-fishes, encrinites, lamellibranchiates, and Orthoceratidae,—a variety o( 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 fos8iliferous 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 due 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 formation, 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 lowsr forms (Myriapods, Ac) only appear as fossils in the succeeding Carboniferous formation. Such highly organized insects also imply the existence oi vegetation, and, by analogy, of other terrestrial animals of an equally high grade of development. Hence the discovery of these winged insects (which can, with great probability, be classed in one of our existing orders—the Neuroptera) 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 Paheozoic '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 islands 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 myriapods, spiders, cockroaches, locusts, dragon-flies, ephemeras, lamellicorn-beetles, and bombyciform moths,—sn 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 geology 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 l sensation as woulJ now be excited by our finding a

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For an account of the characteristics of these small animals, and for some details of their history, we refer the reader to Sir Charles Lyell's work; it is here only necessary to state the circumstances under which these remains have been preserved and discovered. Fossil remains of land animals are, of course, rarely found except in lacustrine or estuarine deposits; and these are often entirely wanting throughout extensive geological formations. But even where such fossiliferous beds occur, the conditions favourable to the preservation of small Mammalia are exceedingly rare,—the entire series of fresh-water Wealden beds having yielded no trace of them, although we are quite certain that they were then both varied and abundant Even more remarkable is the fact that the whole 25 species of Furbeck mammals, belonging to 10 genera, were obtained from a single stratum only a few inches thick, and from an area of less than 500 square yards. Yet these small nnim»1« must have abounded at this period; and it is impossible to believe that anything but a most imperfect and fractional representation of the mammalian fauna of the country could have been gathered into this narrow graveyard. But this thin stratum occurs amid a mass of fresh-water deposits 160 feet thick, the whole of which have been thoroughly and systematically examined by the officers of the Geological Survey of Great Britain; and though many of the layers contain remains of land organisms—plants, insects, and land-shells—no other part of the whole series has yielded a single fragment of mammalian remains 1 Having this striking example of the worthlessness of negative evidence, it behoves as to be cautious of rejecting any legitimate conclusions from the facts lu our possession, on account of the absence- of the direct evidence of fossil remains. The varied and highly

developed Mammalia of the Eocene period roauy necessitate (to the evolutionist) the long-continued previous existence of this class of animals: and the discovery of isolated species in the Oolite and Trias would (had it been delayed to our time) have been but a confirmation of theoretical deductions.

In his anniversary address to the Geological Society in 1870, Professor Huxley adduces a number of special cases showing that, on the theory of development, almost all the higher forms of life must have existed during the Palseozoio period. Thus, from the fact that almost the whole of the Tertiary period has been required to convert the ancestral Orohippue into the existing horse, he believes that, in order to have time for the much greater change of the ancestral Ungnlata into the two great divisions of Perissodactylee and Artiodactyles (of which change there is no trace even among the earliest Eocene Mammals), wc should require a large portion, if not the whole, of the Mesozoic period. Another case is furnished by the bats and Cetacea, which occur fully developed in the Eocene formation; and these would have required still more-time for their modification out - of ancestral Insectivora and Carnivore. Tha Marsupials of the Trias, again; were already differentiated into herbivorous and carnivorous forms; so that on the lowest estimate we must place the common ancestor of the Mammalia very far back in Palaeozoic times. Reptiles famish evidence of the same character. Professor Huxley Bays, "If the very small differences which are observable between the Crocodilia of the older Mesozoic formations and those of the present day famish any sort of approximation towards an estimate of the average rate of change among the Sauropsida, it is almost appalling to reflect how far back in Palaeozoic times we must go before we can hope to arrive at that common stock from which the Crocodilia, Lacertilia, Ornithotcelida, and Pletiotauria, which had attained so great a development in the Triassic epoch, must have been derived." And if to these indications we add the appearance of two orders of fishes—Elasmobranchs and Ganoids—in the Silurian period, we shall be compelled to place the origin of the whole vertebrate stock at an epoch far beyond that of the lowest fossiliferous rocks of the Cambrian series.

If, then, we bear, in mind the very early appearance of so many highly complex organisms) representing all the great types of animal life—almost all the great invertebrate groups in the Cambrian and Lower Silurian, with many Vertebrata and almost all forms of Insecta in the Devonian and Carboniferous periods,—while a large number of these have hardly increased in complexity of organization down to our times, we shall be prepared to admit the extreme probability of Mr Darwin's view, that "before the lowest Cambrian stratum was deposited, long periods elapsed, as long as, or probably far longer, than the whole interval from the Cambrian age to the present day; and that during these vast periods the world swarmed with living creatures" (Origin of Specie*, 6th ed p. 286.)

Professor Ramsay has recently expressed analogous views, founded on an extensive survey of the whole series of geological formations. In a paper "On the comparative value of certain Geological Ages (or Groups of Formations) considered as items of Geological Time" (Proceeding* of the Royal Society, 1874, p. 334), he says—speaking of the abundant and well-developed fauna of the Cambrian period, a sketch of which we hare given at p. 282 :—" In this earliest known varied life we find no evidence of its having lived near the beginning of the zoological series. In a broad sense, compared with what must have gone before both biologically and physically, all the phenomena connected with this old period seem, to my mind, to be of quite a recent description, and the climates of seas and lands were of the very same kind 86 those the world enjoys at the present day.* .

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