very large and was very deep-seated at | We now turn to the changes that have the period of cooling, so that it cooled taken place in the organic beings that slowly and gave off heat for a long time lived upon the globe. This is a subject into the surrounding rocks, these are found that can only be adequately understood to be still more completely changed, having by persons acquainted with botany and acquired a different mineral structure zoology; but we will endeavour to give from what they had when first deposited an idea of the results arrived at. We as aqueous rocks, and different from what must commence by laying down a few any purely aqueous or igneous rocks pos- postulates:--1. A species is a kind of anisess. Granite, for instance, is often found mal or plant, so distinct from all others to be margined by rocks known as Gneiss that the continuation of the species is only or Mica schist, that have a schistose or possible between a pair belonging to that foliated as well as a more or less crystalline species. Offspring cannot be produced structure, and contain minerals such as by a pair of individuals of different species could have been formed only by the agency except when those species are very nearly of heat. We also find these rocks over allied, and then the progeny is barren large spaces where there is no truly either in the first or, at the farthest, in the igneous rock apparent, or not enough of it second generation. It seems to result to be adequate to the widely-spread effect. from this that all the individuals of species They always occur, too, in such situations are the descendants of a single pair.* as to allow of the supposition that they were deeply buried in the earth at some time after their formation, so as to have come within the influence of the heated interior. This would be the case if the area on which they were originally deposited at the sea-bottom became subsequently an area of depression, and if, being thickly covered with other rocks deposited above them, the depression continued long enough for them to reach a low and therefore very hot level in the earth's crust. After this we must suppose them to have again taken their turn as areas of elevation, the rocks to have been lifted up above the sea, and their covering stripped off during the process, in the way we have described. These altered rocks are termed Metamorphic or Transformed -Rocks, 2. Vegetable life subsists upon inorganic food,-matters that may be found in in the earth, the water, or the air, independently of animal life,-while animal life subsists entirely upon organic food, either of vegetable or animal origin. It would follow that no phytophagous animal could continue to live unless vegetable life already existed in sufficient abundance to serve as its food; and no zoophagous animal could exist until there was already an abundance of phytophagous animals. The order of existence, therefore, of organic beings must be-1st, plants; 2nd, plant-eating animals; 3rd, animal-eating animals. 3. Every species of plant and animal has a constitution suited to the degree of heat and light, dryness and moisture, altitude or depth, density of air or water, by which it is surrounded. It is also specially adapted to consume certain kinds of food, which are those most conducive to its well-being. Let us speak of all these circumstances, food included, as its affected by climate; some hardy kinds being able to flourish through considerable variations of it, and therefore to spread over large areas: others, more delicate, being injuriously affected by the slightest modification of it, and therefore confined to small areas, where the peculiar conditions proper to them are alone to be found, and where, if the conditions change, the species declines and perishes. Such have been the agencies at work during all the periods of our chronological scheme. There were always dry lands that were being worn down, always aqueous rocks being formed beneath the seas and lakes, always igneous rocks being in-climate.' Different species are differently truded into the crust of the globe or vomited forth upon its surface, always rocks that were in process of alteration in consequence of their coming within reach of the heated interior, and the coating of the earth was always being raised in some parts and depressed in others, and sometimes fractured and convulsed, in consequence of the reaction of the molten interior upon the consolidated rind. All these operations are still going on, and we have no reason to suppose that either their nature or their intensity was ever, since the commencement of the Paleozoic epoch, materially different from what they are at present. 6 VOL. CVI. 4. Species have come into existence, united in one individual, as in some of the lower *In the case of species in which the sexes are orders of animal life, a single individual may of course have sufficed instead of a pair. here and there all over the globe, at points which we may call their centres, in such a way as to show that similarity of climate by no means involves identity of species; similar climates in different parts of the globe being occupied by different, though often analogous or representative, species, while in many cases, especially when the regions are far apart, the species and even the genera are entirely distinct. Each species has spread from its centre over an area which is measured by its climatic constitution. 6 5. Species of animals and plants may become extinct, either from injurious alterations in climate,' from epidemic diseases, or from their area becoming invaded by rivals powerful enough to exterminate them, or to consume the food on which they previously lived. There may be other causes, but these are all that we know sufficiently to be warranted in assuming them to be effective. difference in the date of their creation, and involves the idea of great lapse of time and a great succession of events as necessary to the production of the existing state of things. All the recent discoveries go to prove that the present laws of distribution in the species of animals and plants are the same that have reigned through every known geological period. In other words, there were always some species so widely diffused as to have been nearly, if not quite, cosmopolitan; and some so narrowly restricted as to have been found only over very narrow areas; always some species just come into existence and struggling for a footing in the world; some at the acme of their power and the full extent of their dominions, which every subsequent change tended to break up and diminish; some long past that point, and fading away in one, two, or more ever-lessening areas; some just on the eve of dying out in their last citadel of retreat. Neither does there appear to be any good grounds for supposing that the rate of the extinction of old species, or that of the creation of new, was ever materially different at one period from its rate at any other. In the absence of all proof to the contrary, it seems most philosophical to suppose that species always died out just as slowly and imperceptibly as they do now. If we assume the present rate of physical change to be the mean rate, and if physical change be the great modifying cause in producing changes in species by that gradual destruction of the old, and that rendering necessary the creation of With these postulates established, coupled with the circumstance that every area that has been yet examined has passed more than once through the alternations of deep sea, shallow sea, low dry land, high dry land and back again, we perceive that the plants and animals that lived upon it must have perished either directly by the physical changes being destructive to them, or indirectly by being fatal to their food, or by laying them open to the incursion of a hostile tribe who either ate up their food or themselves. The alternations in the earth's surface have been so widely spread that all the delicate species of any early period must long ago have ceased to exist, even if any of the hardy ones could have survived. The fact that all the biological provinces of the globe are now crowded, not only with hardy species, which have a great vertical range within the province, and spread perhaps through two or more provinces, but also with numerous delicate and locally-restricted species, specially fitted to peculiarities of climate,' proves that these latter must either have come into existence subsequently to the development of that local climate' for which they are adapted, or must be the last survivors of a race once more widely disseminated when the climate' spread over a larger area. In the first case, they must be of more recent creation than many of their fellow creatures; and in the second, they must be much older than many. The very variety, then, in the life of the globe, and the vast difference that exists in the distribution of species, proves a * The observed facts of geology in some instances seem to warrant the notion of sudden extinction den introduction of new assemblages of species. of whole races of animals or plants, and the sudWe do not intend to deny that many of the fossils found in rocks have perished suddenly, so far as the individuals or the groups of individuals actutend that the notion of the sudden destruction of We do, however, conally found are concerned. whole species is in no case proved, and that the arguments brought forward to prove it are entirely fallacious. The doctrine rests upon the supposition that successive beds of rocks were deposited with of instances this assumption is quite unsupported; but short intervals between them. In the majority in many it can be shown to be entirely erroneous, and that beds, even those to be seen in a single quarry, were formed with indefinitely long intervals between them; while, in other instances, the Our series is in fact but a series of fragments, each, deposition has been rapid and almost continuous, perhaps, representing a few years in itself, but the intervals between them being utterly unknown. If we found a Roman pavement just underneath a modern floor, should we be warranted in supposing that the one was the immediate successor of the other, because there were no other pavements between them? new, in order to keep up the completeness | preceding period survived into the earlier in the life of the globe which seems to be portion of this, and a few into its latter the will of the Creator, we have the two portion. On the other hand, not only are kinds of change so linked together that many species to be found only in the rocks the rate of the one gives us the rate of of the Lower Silurian period, but some the other, and the amount of the one whole genera-especially genera of Trilogives us the measure of the other. bites and Cystidea--seem entirely to have perished before the more recent Silurian period commenced, while other species, forming new genera, were now first created to occupy their place. All the double Graptolites had also died out, though a few single ones survived to the close of this period, when the whole class became Keeping these principles in mind, let us now glance over the following summary of the history of life through the fourteen geological periods specified in our table. Primary or Paleozoic Epoch. 1. Cambrian. In the rocks of the Cambrian period no other distinct and indubitable traces of life have yet been found than the impressions of a zoophyte and the tracks of sea-worms. extinct. therefore belonging to the close of the era, In the uppermost rocks, and there have been found the remains of about a dozen species of fish, and new forms of crustacea make their appearance, more like our present lobsters in external shape, though having many essential points Some of them of difference in structure. 4. Devonian Period. - Fishes abound 2. Lower or (Cambro-) Silurian Period. -In the rocks deposited during this period the remains of animals are often abundant. They consist of the hard parts of Zoophytes, such as Corals, and of a class like the sea-pens (though perhaps of a higher organization), called Graptolites; in some of the rocks deposited in particu a very peculiar order of Echinodermata,*lar areas during this period, many of them called Cystidea, as well as some Stone-of strange forms, with a strong bony arlilies and Starfish. There are several species mour of scales that are sometimes beautiand genera of a peculiar class of bivalve Several of them have shells, known as Brachiopoda, some ordi- been graphically described, in a popular fully preserved. nary bivalves or Conchifera, some ordinary univalves or Gasteropoda, and some of a way, by Hugh Miller, in his Old Red Sandstone,' and their scientific description higher class belonging to the Cephalo-has been drawn up by Agassiz. In other poda. There were also many crustacean animals, some not very unlike our present shrimps in external form, and a great variety of others called Trilobites, of an altogether extinct order of Crustacea. No true Fish, nor, indeed, any vertebrate animal, was then in existence, if we may judge from the negative evidence that no remains of them have yet been discovered; any land plants been as yet found in the rocks of this period. neither have the relics of rocks, believed to be of the same age, there are an abundance of shells, corals, and trilobites. A few of the species of these seem to be nearly the same as those of the upper Silurian rocks, but most of the fossils differ specifically and many generically. Reptiles also existed, more their remains in the beds which now form or less nearly allied to lizards, and left part of the dry land of Scotland. The first fragments of undoubted land plants are found in the rocks of this period, both fronds of ferns and other plants, together in Scotland and Wales, while magnificent with a large shell like a freshwater mussel (or Anodon), have been discovered in the Upper Old red sandstone of the county of Kilkenny. 5. Carboniferous Period.-Plants were deposited in such abundance in the rocks of this period as to form great beds of coal, spreading over many square miles in extent, and occurring at intervals through a series of sandstones and shales, which amount, in some places, to a total *Siluria, p. 264, etc. thickness of at least 8000 feet. In other beds, especially in the limestones, the abundance of marine shells of all kinds (Brachiopoda, Conchifera, Gasteropoda, and Cephalopoda) is quite inconceivable and some of different genera from any of to those who have not seen them. Min- 6. Permian Period.-Another change has now taken place-an almost entire change in the species-although many of them are sufficiently near to their predecessors to be grouped in the same genus. A conspicuous species of Producta, for instance, abounds in some of the rocks. Though it differs from those found in the Carboniferous series so far as to require a different specific name, it is yet obviously a Producta.f Fish of different species, *Even quarrymen are struck with the abundance of these shells, and in some places, with a scarcely pardonable mistake as to the form of the shell, call the rock the oyster shell limestone. Lions, tigers, leopards, panthers, wild and do mestic cats, however unlike in size, in colour, in variety of marking, and so on, have yet evidently so much in common that we naturally group them together as animals of the Cat kind, differing in deed in species among themselves, and yet forming but one genus (the genus felis,' or 'cat') when compared with all other animals, such as dogs, wolves, bears, &c. The specific difference and generic resemblance with which we are familiar in the above example reigns throughout the animal and vegetable kingdoms, and is just as real and important among cockles, limpets, oysters, starfish, and corals, as among creatures of the highest organization. remains that they respectively contain. It does not follow that the changes may not in reality have been as gradual at this as at any other part of the world's history, since the seeming abruptness may be the result merely of the juxtaposition of two sets of things, the dates of which were widely separated in time without any substantial record of the interval that elapsed between them. Secondary or Mesozoic Epoch. 7. The Triassic Period.-In Britain confined to fragments of fossil wood, and our records of life during this period are a few relics of bones, and tracks on sandstones of huge Batrachian and other reptiles similar to those that are found in the Permian rocks. In Germany, however, and on both flanks of the Alps, rocks are found crowded with the remains of marine creatures, part of which have an essentially peculiar character, while others are more allied to Palæozoic forms. Some of the genera become more fully and largely developed in future periods.* Many of these fossils are evidently fragments of the missing chapters of our history. 8. Oolitic or Jurassic Period.-The British rocks of this period are the most complete type of a great geological 'formation with which we are anywhere ac * Lyell's Supplement, p. 25. quainted, whether we look to the mere rock groups or to their fossil contents, and are only to be approached by the great Carboniferous formation of our islands. They are divisible into five or six groups, each of which can be as clearly subdivided into two or more sub-groups. Each main group has a whole assemblage of fossils, of almost all kinds, peculiar to itself; while each of the sub-groups has also its smaller but still very distinct as semblage of species, which are either never, or very rarely, found in any group above or below. An old Palæozoic group of corals has become extinct with all its genera and species, and been succeeded by others belonging to orders that still exist. The Brachiopodous class of bivalves has diminished in importance to something more like its present proportions, while Conchifers and Gasteropods have become more numerous. Cephalopodous chambered univalves, which in Paleozoic times were developed in strange forms of Orthoceratida (like Nautili unrolled and pulled straight), and in sharplytoothed Goniatites, now appeared as Ammonites, with their hundreds of varieties, while the intermediate genus Nautilus, commencing in the earliest ages of the world, has continued to appear in species after species down to our own time. The old forms of fish had vanished, but were replaced by many new kinds, the majority of which, like the most numerous fish of the present day, had equal-lobed tails; while all Palæozoic fish resembled our sturgeons and sharks, in having the rays of both lobes of the tail wholly underneath the termination of the vertebral column. The most striking fact connected with the life of this period, however, is the amazing development of reptile forms. Thousands of huge Ichthyosauri breasted the ocean, and thousands of equally huge Plesiosauri lurked along its shores. The land was tenanted by immense Megalosauri, and even the air became peopled by flying dragons in the shape of winged lizards, with long jaws and sharp teeth, known as Pterodactyles. Nor was even the highest class of the animal kingdom, the Mammalia, unrepresented, since portions of the skeleton of several Marsupial and other 'quadrupeds' (as they used to be called) have now been disinterred from rocks belonging to this period. In the Portland and Purbeck group, which is the uppermost or newest of the oolitic series, we 13. also get vegetable soils with broken stools and prostrate stems of trees preserved in a fossil state. 9. Cretaceous Period.-The fact of our finding land plants and land animals in the upper oolitic rocks of Britain, shows that there must have been dry land somewhere in the neighbourhood during the latter part of that period. There is therefore nothing very unexpected in the fact that the earliest record of the succeeding period which we meet with in Britain, is a fossil delta of a large river-a delta as great apparently as that of the Nile or the Ganges. The beds of this delta, which in some places exceed 1000 feet in thickness, contain fresh-water shells, drift-wood, and the bones of strange terrestrial herbivorous reptiles of enormous bulk, which have received the names of Iguanodon and Hylæosaurius. Such a delta, evidently the sweepings of a large river, involves the necessity of a large and perhaps a continental land; nevertheless the delta is covered by accumulations of other rocks, all crowded with the remains of marine animals, to a depth exceeding 2000 feet, and spreading far and wide on all sides over and around it. The chalk, which is the uppermost of these accumulations, is alone in some places 1000 feet thick. In the marine fossils, so numerous in the upper rocks of the Cretaceous period, there is again a total change in species from those found in the Oolitic rocks below. There were Ammonites and Belemnites, and Terebratulæ and other shells, with common generic names in both eras; but those found in the Cretaceous rocks are obviously different from those found in the Oolitic. So with the Echinodermata, so with the Fish, so with the Reptiles, so with all other classes and orders of animals and plants, so far as their remains have as yet been described. At the end of the Cretaceous period we once more meet with a sensible gap in our series of records, and a corresponding change in the characters of organic beings, after we have passed it. Here, therefore, we draw another strong boundary line and close this second book of our history, and open that of the Tertiary or Cainozoic epoch, which includes that of our own days. Whoever examines a museum* contain * In London we may point to the Museum o Practical Geology and that of the Geologica Society; there are also excellent museums, with fossils chronologically arranged at Cambridge, at * Lyell's Elements, p. 312, and Supplement, p. Oxford, in Bristol, Liverpool, Manchester, York, and other large towns in England; at Edinburgh |