# Evolution of Life/Geology

GEOLOGY.
No study illustrates better than Geology not only the advantage but the absolute necessity of general knowledge for the thorough understanding of any particular subject. Geology means literally a discourse on the earth. The student of so vast a theme ought naturally therefore to be familiar with at least the general conclusions offered by Astronomers, Physicists, Chemists, Mineralogists, Botanists, and Zoologists, so far as they relate to the history of this planet, since by astronomical data we picture our earth as a once gaseous, chaotic mass. The study of the cooling of heated bodies underpressure, implying a knowledge of the laws of Heat and Chemistry, furnishes the clue to the explanation of the origin of many formations. Mineralogy distinguishes the different rocks of which the crust of the earth is composed, while Botany and Zoology supply the means by which the life of bygone days is revivified, enabling us to interpret the structure and relations of plants and animals long since extinct. Geology, therefore, is not a separate science, since it consists only of the conclusions of many sciences applied to the investigation of the past and present history of the earth. As three-fourths of the earth are covered with water, with our present resources only a very small portion is susceptible of geological examination. And notwithstanding the great number of surveys and scientific expeditions which have been made during the present century, with the exception of Great ​Britain, Canada, parts of Europe and the United States, the geology of the accessible portion, even, of the earth is still very little known, large parts of Africa, Asia, and South America being as yet, comparatively speaking, unexplored. Civilization, through its railroad-building, tunneling, canal-making, and mining operations, furnishes a large amount of the material on which the Geologist bases his science. Through agencies of this kind, rocks have been exposed which otherwise would have perhaps remained forever concealed from view. Through the excavating incidental to mining and tunneling, there have been discovered the remains of plants and animals long since extinct, the relics of an indefinitely remote past, the existence of which had not been previously even dreamed of The detritus brought down by rivers, and the consequent filling up of their mouths, as seen in the deltas of the Mississippi and the Nile, with the preservation in the mud, etc. of the coral stones, shells, skeletons of fish, etc. which lived and died in the vicinity, give one a good idea of the manner in which petrified organic remains or fossils may have been preserved in the rocks. While in certain rocks of this kind the fossils are found in great profusion and in a very excellent state of preservation, in others very few occur, or only a fragment may have escaped destruction. This is often, however, so characteristic that the comparative anatomist can reconstruct the whole skeleton from a single bone, a knowledge of the correlation of forms enabling the osteologist to infer from the structure of the foot the nature of the jaws, teeth, etc., of the extinct animal. Many such inferences might be mentioned, all of which, while commanding praise as illustrating the osteological knowledge of the anatomist, scarcely merit the astonishment which they invariably excite. While many rocks seem to have experienced but little disturbance since their original deposition, the different layers or strata of which they ​are composed being easily distinguishable, the convulsions to which others have been subjected have been so great, and the effects of heat so intense, that no sign of such stratification is visible, if it ever existed. The old Geologists resembled the knights who fought about the color of the shield. The early German school, influenced by the character of the rocks in that part of the world, attributed a great deal to the action of water; while the Scotch school, equally impressed by the features of the formations in their country, attached great importance to the effects of heat. Hence arose the sects of the Neptunists and Plutonists. Both were right in attributing the formation of the rocks in their respective countries to the action of water and heat. Both were wrong in applying to the whole world conclusions drawn from such local data. Modern Geologists steer a middle course,—avoiding these extremes,—considering the effects of the combined action of water and heat, as well as admitting the influence exerted by these agents separately. Rocks the origin of which is supposed to be due to the gradual deposition under water, in layers or strata, of the materials composing such formations, are called Aqueous Rocks; while those of which the structure clearly testifies to the action of heat in producing them are known as Plutonic Rocks. Finally, the Metamorphic Rocks illustrate the alternate action of water and heat. Geologists classify rocks according to their mineral composition, their organic remains, and the order in which they follow or overlie one another. The Geologist, starting in Canada, and traveling through New York and Pennsylvania, notices continually as he advances southward the change in the minerals composing the rocks, and the different aspects of their organic remains. Thus, in Canada and the east of New York, granite, gneiss, and syenite are common minerals. These rocks were originally called Azoic, or without life; improperly, however, as within a few years the Eozoon, or ​Morning being, was discovered, so called from representing the very simple beings which first appeared on our continent during the dawn of life. The term Azoic is still retained by Geologists as meaning a scarcity of life, not implying, as formerly, entire absence of it. Continuing his journey, our Geologist soon reaches the Potsdam Sandstone, abounding in characteristic fossils, of which the Brachiopod shells and Trilobites, extinct Crustaceans, are very common. Passing by the town of Oriskany, he comes to what is known as the Schoharie Grit, in which the remains of fishes are first found. Finally, he reaches Pennsylvania, abounding in coal, with its ferns and traces of reptiles. If he goes over to New Jersey, he finds in the chalks the reptiles more abundant. To see the higher forms of life in profusion, he must turn to the West, Nebraska and Dakota having furnished the remains of deer, rhinoceros, hyena, lion, etc. The rocks of Canada and New York are the successive beaches left dry by a retreating ocean. This is very evident from the manner in which the rocks follow each other, the ripple-marks still visible on them, and their marine remains. The animals and plants found in the rocks of New York and Pennsylvania, from bearing the stamp of age, are called Paleozoic, or old beings, and the age in which they lived is known as the Primary. The fossils of New Jersey, though still old, are more modern in their appearance than those of New York; they are called, therefore, Mesozoic, or middle-aged beings; the rock containing them forming, with some others not well represented on our continent, the Secondary Age. The Cenozoic, or recent beings, those of Nebraska, for example, lived during the Tertiary Age. Not only are the fossils invaluable to the Zoologist and Botanist, as representing the life of the past, but they are equally important to the Geologist, as we have just seen; his classification of the rocks is principally based on the extinct remains which they contain. The ​Primary, Secondary, and Tertiary Ages, with their characteristic fossils, according to many Geologists, were not confined to America, but extended all over the globe; the whole earth having passed at the same time successively through the Primary, Secondary, and Tertiary Ages. Some few Geologists do not accept this onion-coat hypothesis, which supposes that similar rocks, with similar remains, were deposited at the same time all round the earth, like the layers in the coat of an onion. With all deference to Geologists, let us examine the tests used for determining the time of the deposition of foreign rocks as compared with our own. The test of having similar minerals, when applied to, elucidating the age of foreign rocks as compared with those on this continent, is worthless, since the chalk, sandstone, etc. of which the rocks are composed are forming in all ages; while a determination of the age of rocks, based on the order in which they follow or overlie one another,—when applied, for example, to New York and England, separated by an ocean,—to say the least, is very unreliable. The third test, that rocks having similar organic remains are of the same age, considered by most Geologists as settling the question, whenever such comparisons are possible, may be as fruitful a source of error as the view that similar minerals deposited in the same way are of the same age. Nor does the reverse of this proposition hold good, that rocks are of a different age because they contain different fossils. Suppose, for example, that the western part of North America and Australia were gradually to sink into the sea, as parts of the world are now doing, and then slowly to rise again, the Geologist of an indefinitely remote future might argue, because he found many fossil pouch-bearing animals in Australia, and the bones of an extinct human race in America, that the Kangaroo was not contemporaneous with the Indian. From the distribution of plants and animals at the present time, we know that ​remote parts of the earth have very different animals; the preservation of organic remains at the mouth of the Mississippi being no index of what is going on at the mouth of the Ganges. While it is possible, it is certainly not proved by the structure of the rocks, their deposition, and organic remains, that the whole earth has passed at the same time successively through the Primary, Secondary, and Tertiary Ages. The limits of this essay do not admit of the further discussion of this subject; nor, indeed, is it necessary, as the question has been thoroughly argued by Herbert Spencer in his "Illogical Geology." The disputes of the Neptunists and Plutonists ought to be a warning to Geologists not to apply generalizations, drawn from limited data, to the whole earth. In the present state of Geology we should receive all conclusions with great caution, being prepared at any moment to have them modified or even disproved by future research. Notwithstanding the difficulty of obtaining fossils, the injuries they often have received in being removed from the rocks, that many are lost or destroyed through the ignorance of the workmen who are often the first to find them, together with the fact that the chance of plants and animals being preserved is very small, remembering how the remains of an animal, dying at the present day, are picked to pieces, get separated, and are often finally destroyed,—yet the museums in different parts of the world contain numerous organic remains on which is based the science of extinct plants and animals, or Paleontology, the conclusions of which science are important proofs of the truth of the theory of the evolution of the higher forms of life from the lower. The opponents of the transmutation of species argued fifty years ago. If the higher forms have descended from the lower, where are the missing links? Paleontology has answered that objection by supplying the missing links, such as the intermediate forms which bind together the Rhinoceros ​and the Horse, the Hippopotamus and the Pig, the Whale and the Seal, the Reptiles and Birds, the Ganoid fishes and Batrachia, etc. Not only are the fossils invaluable, therefore, to the Evolutionist without reference to their age, but the order in which they have appeared, and their relative age so far as it is possible to determine it, are in perfect harmony with the conclusions we have drawn from the structure of living plants and animals. Remembering the uncertainty attached to the absolute and relative age of rocks, let us examine the Primary, Secondary, and Tertiary Ages through which North America has probably successively passed, without reference to the relation these Ages bear in time to the corresponding parts of Europe, etc. Geologists subdivide the Primary, Secondary, and Tertiary Ages into periods (epochs) more or less characterized by their fossils.
AGE OF MOLLUSCA AND ALGAE.
Passing from the Azoic rocks, in the northern part of the State of New York, through the Potsdam region, to Trenton Falls, southwardly to the Helderberg Mountains near Albany, and eastwardly to Niagara, the immense number of fossil shells, particularly Brachiopods (Fig. 145), attracts the attention of the traveler. The Brachiopods of the present seas are few and far between, whereas the sea of that most ancient period was characterized by shells of this order; the remains of other Mollusca are found, but much less abundantly as compared with those of Brachiopods. The seas of this period must have swarmed with Crinoids, from the great number of them found petrified, their broken stems being known as Lily Stones (Fig. 146) and St. Cuthbert beads. The young of the Comatula, long supposed to be a distinct animal, the Pentacrinus (Fig. 42), is the only known representative of the Crinoids at the
​Star-fish. Associated with Brachiopods, etc., in great profusion, and in an admirable state of preservation, are the characteristic Trilobites (Fig. 144), an extinct order of Crustacea, to which the nearest approach at the present time is seen in minute Crustaceans like Cypris, favorite objects with the Microscopist, or like the larva of Limulus. In some genera of Trilobites, the different stages of their existence have been very well followed out, the fossils having been found perfect and in great profusion. The rocks furnish evidence of the existence of worms at this period, though, from the delicate nature of their bodies, their remains are few and obscure. Certain impressions or casts found in these rocks, known as Graptolites, are supposed to have been made by animals allied to the Sertularia of the present day, while the Niagara limestone consists almost entirely of Coral. The period characterized by the profusion of the remains of Brachiopods, Crinoids, and Trilobites is known as the Silurian, called after that of England and Wales, which derived its name from the ancient tribe of Silures, once inhabiting those parts. The plants of this period are Fucoidæ, or brown sea-weed. What conclusions can be drawn from the life of the Silurian period in favor of the theory of the higher forms having descended from the lower? We have seen that the animals of this period were aquatic. Now, animals living in the water are more simply and lowly organized than those living on land. An animal subjected to the ever-changing conditions of a land-existence needs a more complex organization to fulfill its requirements than one living in the comparatively unchanging sea. If the Development theory be true, the water-animals, then, should have preceded the land-animals, the water-plants the land-plants. Such has been the order of their appearance, according to the testimony of the rocks. In the chapter on Zoology we gave reasons for supposing that the Crinoids and ​were the oldest of the Echinodermata, the Brachiopoda of Mollusca, the Entomostraca of Crustacea, and that the Worms preceded the Insects, etc. We have just seen that the life of the Silurian, the most ancient period except the Azoic, was characterized by these very orders, which are the most simply organized of the respective divisions of the animal kingdom, while the Fucoidae, or brown sea-weed, found fossil in rocks of this period, belong to the Algae, the simplest division of the vegetal kingdom. Geological evidence confirms, therefore, not only in a general way, but to an extent in detail not to be hoped for from the nature of the subject, the view of the development of the animal and vegetal kingdoms deduced from their structure. The Silurian period is sometimes called the Age of Mollusca and Algae.
AGE OF FISHES.
Passing from the Silurian period to the Devonian, so called from the rocks of this formation having been first studied in Devonshire, England, we notice that while the first half of the Devonian agrees in its main features with the latter half of the Silurian, the latter half of the Devonian, often called the Old Red Sandstone, offers evidence of a progress in life, since its rocks contain the remains of fish, together with a few Ferns, Lycopods, and Conifers. The remains of these plants are, however, only rarely found in the Devonian; the flora of this period, as well as that of the Silurian, being more generally characterized by the presence of Algae. The Fishes found in the Devonian period are Sharks and Ganoids (Fig, 147). The Sharks belong to the order of Cestraphori, or weapon-bearers, so named from their dorsal fin being armed with a long spine; these spines ​are found fossil in great numbers; the teeth are in the form of plates, giving the appearance of a pavement. The only Shark at the present day having such teeth is the Cestracion, or Port Jackson Shark, confined to the Australian and China seas. The Ganoids, so called from their shining plates or scales, must have abounded in the Devonian seas, from the numerous fossil genera and species that have been described. The only living examples of Ganoids at the present time are the Sturgeons, Gar-pike, Amia of North America, and the Polypterus of the Nile. In the chapter on Zoology we argued, from their structure, that the Sharks and Ganoids were not so highly organized as the Teliosts, or bony fish of the present day, and concluded that therefore the Sharks and Ganoids had preceded the Teliosts in their appearance on the earth. This view is confirmed by what we have just seen, that the fishes that first appeared were Sharks and Ganoids. Further, we noticed that the Ganoids, while intermediate in many respects between the Sharks and Teliosts, have many striking affinities with the Batrachia and Reptilia. The fact of the Ganoids appearing before the Bony Fish and Batrachia is a striking confirmation of the truth of the view proposed, that the Ganoids were the common stock from which the stems of the Teliosts and Batrachia diverged. Calling attention to the fact of the Silurian period, or Age of Mollusca, preceding the Devonian period, or Age of Fishes, being in harmony with the view of the higher forms of life coming from the lower, we pass on to the Carboniferous period.
AGE OF ACROGENS AND BATRACHIA.
Pennsylvania, the great coal State, was principally formed during the Carboniferous period, often called the Age of Acrogens or Summit-growers, from eight-tenths of its plants belonging to that order of the vegetal kingdom. ​Some years ago it was estimated, by Brown, that of the one thousand species of plants found in the rocks of the Primary Age, especially of the Carboniferous period, not less than eight hundred and seventy-two were Fern-like, the remaining species including about seventy-seven Coniferæ and Cycadæ, forty Thallophytes, mostly Algæ, and about twenty undetermined plants. We see from this estimate that the Fern-like plants were the characteristic feature of the Carboniferous period, and must have flourished in a much greater profusion than at the present day, the Tree-ferns of tropical climates, even, giving one no idea of the luxuriance of their growth in those ancient days. Indeed, whole orders have passed away: the Calamites and Asterophyllites, resembling the Horse-tails, having no living representatives, while the Sigillariæ and Lepidodendrons have degenerated into the Club-moss of our forests. As commonly known, the Lycopod of the woods is a delicate moss-like plant: that of the Sunda Islands is often twenty-five feet high. The Lepidodendrons of the Carboniferous period, closely allied to living Club-moss, attained, however, a height of from forty to sixty feet, while their diameter at the roots was as much as twelve to fifteen feet. The Sigillariæ are similar in many respects to the Lepidodendrons, often as high, though more slender. The general aspect of the Carboniferous period was that of a great Fern forest and a jungle of gigantic Club-mosses, with some Coniferæ and Cycadæ; these, however, but rarely seen, comparatively speaking. The gradual decomposition of these plants resulted in the formation of the vast coal-fields so characteristic of this period. In the marshes of these forests first appeared the Batrachia (Frogs, etc.), together with the Centipedes, the May-fly, Locust, and Beetle orders among Insects. We see, therefore, that the tree of the development of life, as proposed in the chapters on Botany and Zoology, is in perfect ​harmony with what we know of the Carboniferous fossils. The gorgeous Ferns, in great variety, the Lycopod-like plants, having attained the full maturity of their luxuriant growth after this period, give way to the Cycadæ and Coniferæ. The Ganoid fishes die out, their posterity, the Batrachia, having appeared, soon to be replaced, however, by the Reptilia, while the Insects are still represented by the lowest orders just mentioned. Following the Carboniferous rocks of the West in this country, and closely resembling the Carboniferous period in its general features, we meet the Permian, called after the ancient kingdom of Permia in Russia. It is interesting to the Evolutionist as furnishing the remains of the simplest reptiles, the Proterosaurus having been found in the Permian rocks of Germany. By looking at the tree of the development of the Reptilia, it will be seen that the Proterosaurus is regarded as the common ancestor of that group. The Silurian, Devonian, Carboniferous, and Permian periods, taken together, constitute the Primary Age, or age of most ancient beings.
AGE OF CYCADÆ AND REPTILIA.
The Secondary, like the Primary Age, is subdivided into three periods, the Triassic, Jurassic, and Cretaceous. These three periods, while differing considerably in minor respects, agree essentially in their plants and animals, being principally represented by Cycadæ and Reptilia.
TRIASSIC PERIOD.
This period derives its name from the formation in Germany being composed of three kinds of rock; the name, however, is one of only local application; the period being often called in England and America the New Red Sandstone, as distinguished from the Old Red, or Devonian. The absence of Lepidodendrons and Sigillariæ in this period, ​so striking a feature of the Carboniferous, is to be noticed as an important fact for the Evolutionist, the Cycadae and Coniferae completely replacing them. Among the Batrachia of the Trias are to be noticed the immense Labyrinthodons, the skull in one species measuring three feet long by two wide; remains of these animals have been found near Gwynedd, in the Triassic of Pennsylvania. The Connecticut Sandstone is famous for its tracks, supposed to have been made by large Reptiles, of which more than fifty species have been described. During this period the Birds first appeared, and, from their tracks left in the sandstone, they are thought to have resembled the Running-birds of the present day, though much larger, the Brontozoon (Fig. 148) of the Connecticut valley being four times as large as the Ostrich. The existence of such large birds may be doubted by those who are not familiar with fossils. Those, however, who have seen the gigantic Dinornis of the British Museum are quite satisfied with Prof. Owen's statement, that they are "equal to the formation of tridactyle impressions as large as those of the Connecticut Sandstones." (Pal., p. 331.) In the Triassic the remains of Mammals are first found. The fossil remains Microlestes and Dromatherium (Fig. 149) are usually regarded as Marsupials, or pouch-bearing Mammalia. The Dromatherium, according to Prof. Owen, "would appear to find its nearest analogue in the Myrmecobius," a little Marsupial living at the present day in Australia. These fossil Mammals have been supposed by some authors to be Monotremata, though that order have no teeth, as in the Duck-bill, etc. This perhaps, however, was not the primitive condition of the order, the first Monotremata having teeth, which their descendants have lost through adaptation to their peculiar mode of life. Whether this view be or be not confirmed by future research, the important fact to be noticed is that in either
​the lowest of the order. The existence of such large birds as the Brontozoon, at this period, is in harmony with the view of the Reptiles being the progenitors through the Ostrich family of the Birds, while the fact of both Birds and Mammals appearing about the same time confirms the theory that they are the diverging stem of a common stock, the Reptilia.
JURASSIC PERIOD.
This period is called after the Jura Mountains of Switzerland, and is remarkable for the variety of its Reptiles, which were of great size. Conspicuous were the "terrible reptiles," or Dinosauria, of which the carnivorous Megalosaurus and herbivorous Iguanodon were upwards of thirty feet long. Very curious flying Reptiles existed in the Jurassic period, such as the Pterodactyle and Dimorphodon (Fig. 150). Equally characteristic were the Ichthyosauri (Fig. 58) and Plesiosauri, upwards of thirty feet long, whose organization united reptilian with batrachian and piscine characters. Their fin- or paddle-like extremities would indicate that they had diverged from the stem of Fishes rather than from that of the Batrachia. Their structure, however, is so peculiar as to make it extremely difficult to determine their exact position in the animal kingdom. Crocodiles and Turtles appear now for the first time, together with Sharks of the cutting-teeth kind, like the modern gray Shark (Notidanus), which will soon replace the Cestracions, so striking a feature of the ancient formations, while the Insects are represented by the high order of Hymenoptera. The Compsognathus (delicate jaw), a very bird-like Reptile, and the Archeopteryx (ancient bird), a reptile-like Bird, both Jurassic fossils, are extremely interesting to the Evolutionist, as almost bridging over the gap between existing reptiles and birds. The Mammals of this period are still of the Marsupial order.
CRETACEOUS PERIOD.
This name is given to rocks occurring in various parts of the world, which contain well-marked and characteristic forms of animal and vegetal life, though the rocks themselves may be composed of very different minerals. Thus, the chalk-cliffs of England are so striking as to give her the name of Albion, while up to the present time no chalk has been found in America. The formation in New Jersey, etc., supposed to correspond to the Cretaceous of England, consists principally of marl, much used for fertilizing purposes. The apparently simple and generally unobserved phenomena of one's fireside are often really so complex that lives have been spent in investigating, volumes written in explaining them. The burning of a candle forms the subject of an interesting little book by the late Prof. Faraday; while Prof. Huxley observes, "The man who should know the true history of the bit of chalk which every carpenter carries about in his breeches-pocket, though ignorant of all other history, is likely to have a better conception of this wonderful universe, and of a man's relation to it, than the most learned student who is deep-read in the records of humanity and ignorant of those of Nature." It would be superfluous to attempt to show the justice of this profound remark, as those who care to follow the reasonings by which such a conclusion is reached can find them in the essay on a "Piece of Chalk," from which the above quotation is taken. While the Reptiles of the Cretaceous period still include huge creatures like the Hadrosaurus and Mososaurus, the Fishes and Plants are becoming more modern in their appearance. Bony Fishes first appearing in this period, among which are to be mentioned the Herring, Salmon, etc., and the vegetal kingdom being represented by modern trees, like the Palms, Oaks, and Poplars, accompanied by a marked decline in the Cycadae. With the ​Cretaceous period we leave the Secondary Age, and pass on to the Tertiary.
AGE OF PALMS, EXOGENS, AND MAMMALS.
The Tertiary Age is subdivided into the Eocene, Miocene, and Pliocene periods. These names were chosen to express the result of a comparison made between the shells found in the rocks of the Tertiary formation and those living at the present day, the object in view being to determine whether many living shells are found petrified in the Tertiary rocks. Thus, in Sicily, of one hundred petrified shells, from seventy to ninety are found in existing seas; hence the name Pliocene, or most recent, was given to rocks containing such a large proportion of living shells. Those parts of the Tertiary formation known as Miocene, or less recent, have from forty to fifty per cent., while only the dawn of recent shells is expressed by the term Eocene. The subdivision of the Tertiary Age into these three periods, originally based on the proportion between the fossil and living shells, was afterward applied to Tertiary plants and animals generally, it being supposed that a proportion similar to that of fossil and living shells existed between Tertiary plants and animals and those of the present day. These periods often, however, pass so gradually into one another, the lines of demarkation not being very well defined, that this classification is not always applicable. The Tertiary Age, notwithstanding the minor differences of its periods, is essentially an age of Mammals, Palms, and Exogens. There is no necessity of describing the details of its animal and vegetal life, since Asia and Africa, with their Hippopotami, Rhinoceroses, Elephants, Lions, and Tigers, living amidst the characteristic tropical plants give one an excellent idea of what America, Great Britain, etc. were during their Tertiary Ages. To the Evolutionist the ​Paleotherium (Fig. 151) and Anoplotherium (Fig. 152), living during the early part of this age, are extremely interesting, being regarded as the progenitors of the odd- and equal-toed Mammalia. The conclusions of Cuvier as to the nature of the Paleotherium, based only on fragmentary remains, were perfectly confirmed by the discovery of an almost entire skeleton. Since that time many allied forms have been described, principally by Prof. Owen, some of which, uniting the Rhinoceros, Tapir, and Horse, make the group of odd-toed, while others, associated with the Hog, Hippopotamus, etc., form that of the equal-toed. Prof. Leidy has described many kinds of horses found fossil in the western part of the United States, etc. (these discoveries are confirmed by those of Owen and Rutimeyer), which represent the transient stages through which the modern horse passes, so that the descent of the Horse from some paleotheroid form is completely made out. As regards the Flora of the Tertiary Age, as compared with that of the Cretaceous and Modern periods, according to Brown, the Apetalae (Fig. 133) were greatly in excess during the Cretaceous period, the Diapetalae were represented by a few species, while the Gamopetalse (Fig. 151) had not appeared. In the Tertiary. Age the Diapetalae exceed the Apetalae, the Gamopetalas being comparatively well represented; while at the present day the great number of Gamopetalous genera seems to indicate that this order of plants is increasing most rapidly. These facts are very significant when compared with what is said of the structure of these plants. The age following the Tertiary, that in which we live, is known as the Age of Man, whose early condition, etc. will be treated of in the chapter on Anthropology. Repeating that great caution must be exercised in accepting the generalization of Geologists as to the relative and absolute age of rocks, a
​progress of the higher forms of life from the lower. The Brachiopods, the lowest of Mollusca, the Crinoids and Star-fish, the lowest of Echinodermata, and the Trilobites, among the lowest of Crustacea, abounded in the Paleozoic Age. The Crinoids and Brachiopods lived on through Secondary time, playing, however, an inferior role, and now have almost passed away, a few Brachiopods only and one Crinoid living at the present day. The Age of Mollusca, we have seen, was followed by an Age of Fishes, thus exhibiting a progress in the animal life of the globe. The fact of these fishes being Sharks and Ganoids is very significant: the important point to be noticed is, that whatever view be taken of the rank of the Ganoids among fish, they preceded the Teliosts and Batrachia, and that the Sharks with pavement teeth came before those with cutting teeth. The next two periods offer a further progress in the life of the globe, since we find the Batrachia (Frogs, Labyrinthodons) appearing in the Carboniferous, followed by the Reptiles (Proterosaurus) in the Permian, while the Insects are represented by the lower orders, of which the Neuroptera (May-flies) were very abundant in the Carboniferous. In the Secondary Age the Reptiles reach their climax, while the Bony Fishes, Mammals, and Birds are just appearing. The gradual unfolding of the vegetal kingdom during the Primary and Secondary Ages is as marked as that of the animal kingdom. An Age of Algae was followed by an Age of Acrogens; these gave way to Cycadae and Coniferae; the Cycadae, in their turn dying out, were replaced by the Palms of the Tertiary, associated with which are the Forest trees, among which the great Mammals lived, and the flowering plants offered then as now a resting-place for butterflies, which first appeared in this age.
Modern Geologists do not believe that life, since it first appeared, has ever been extinct all over the globe at the ​same time. The earth has, no doubt, from time to time experienced great changes, its life being more or less destroyed by the effects of earthquakes, volcanoes, etc. These catastrophes were, however, local in their effects, as at the present day. If living plants and animals be compared with those whose remains have been preserved in the rocks, we see that, while many species and genera have passed away, comparatively few orders have become extinct,—that is, there are very few fossils which have not their modern representatives. Further, where the rocks overlie or follow each other, plants and animals appear in the later formation which did not pre-exist in the earlier, and usually exhibit a more complex structure. So that the "persistent types of life" seem to have been more or less modified from time to time. These general conclusions are in perfect harmony with the doctrine of the gradual development of the higher forms of life from the lower. We turn now to Embryology, which confirms, in the most remarkable way, the tree of life deduced from the structure and fossil remains of the animal and vegetal kingdoms.
CLASSIFICATION OF ROCKS ACCORDING TO THEIR ORGANIC REMAINS.
| TIME. | PERIOD. | PERIOD. | AGE. | AGE. |
| Primary. | $\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \ \end{matrix}}\right.}$ | Silurian. | $\scriptstyle {\left\{{\begin{matrix}\ \end{matrix}}\right.}$ | Mollusca, Algæ. |
| Primary. | $\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \ \end{matrix}}\right.}$ | Devonian | $\scriptstyle {\left\{{\begin{matrix}\ \end{matrix}}\right.}$ | Fishes. |
| Primary. | $\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \ \end{matrix}}\right.}$ | Carboniferous. | $\scriptstyle {\left\{{\begin{matrix}\ \end{matrix}}\right.}$ | Batrachia, Acrogens. |
| Secondary. | $\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \ \end{matrix}}\right.}$ | Triassic. | $\scriptstyle {\left.{\begin{matrix}\ \\\\\ \ \end{matrix}}\right\}\,$ | Reptiles, Cycadæ. |
| Secondary. | $\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \ \end{matrix}}\right.}$ | Jurassic. | $\scriptstyle {\left.{\begin{matrix}\ \\\\\ \ \end{matrix}}\right\}\,$ | Reptiles, Cycadæ. |
| Secondary. | $\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \ \end{matrix}}\right.}$ | Cretaceous. | $\scriptstyle {\left.{\begin{matrix}\ \\\\\ \ \end{matrix}}\right\}\,$ | Reptiles, Cycadæ. |
| Tertiary. | $\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \ \end{matrix}}\right.}$ | Eocene. | $\scriptstyle {\left.{\begin{matrix}\ \\\\\ \ \end{matrix}}\right\}\,$ | Mammals, Palms, Exogens. |
| Tertiary. | $\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \ \end{matrix}}\right.}$ | Miocene. | $\scriptstyle {\left.{\begin{matrix}\ \\\\\ \ \end{matrix}}\right\}\,$ | Mammals, Palms, Exogens. |
| Tertiary. | $\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \ \end{matrix}}\right.}$ | Pliocene. | $\scriptstyle {\left.{\begin{matrix}\ \\\\\ \ \end{matrix}}\right\}\,$ | Mammals, Palms, Exogens. |
| Quaternary. | $\scriptstyle {\left\{{\begin{matrix}\ \end{matrix}}\right.}$ | Modern. | $\scriptstyle {\left\{{\begin{matrix}\ \end{matrix}}\right.}$ | Man. |