CHAPTER IV.

CAUSES OF EXTINCTION OF ORGANIC LIFE.

WHEN the paleontologist has completely established his position, that all the organic phenomena of primeval times have resulted from the impress of original structure, in opposition to the theory of progressive development and transmutation of species; and when he can trace, also, corresponding changes in the mineral formations in which the fossil remains are imbedded, the important inquiry has still to be made into the causes of the extinction of so many races of the animal and vegetable kingdoms. Introduced successively upon the surface of the earth, was there always a physical and necessary relation betwixt the living tribes and the varying conditions of the surrounding media in which their lot was cast? And do the differences in the one explain the changes in the organic functions of the other?

When we look back to the earliest of the fossiliferous rocks we can discover something in the nature of their materials themselves which would cause the destruction of their organic tribes. The Silurian strata have been violently disturbed, and much molten matter, during the period of their deposition, injected among them; and by causes such as these, life would suffer greatly, and whole races be suddenly destroyed. Even the strong incased ganoids and placoids of the Devonian period could not always be able to subsist and bear up against the spasmodic throes that produced the conglomerates. Animals preyed likewise upon each other, and by this means kept up then, as now, the general average and balance of life. But in none of these modes can anything like a LAW be inferred, any stated provision be detected, for the outgoing and the incoming of the different genera and species which successively peopled the globe. The rocks differ, as the organ

isms differ, age to age, from each other: but the series of changes traced in the one class of phenomena, furnish only a few data by which to determine as to the alterations that would be produced in the class cotemporaneous.

No land animals have been found in any of the formations beneath the new red sandstone. No quadrupeds existed before the tertiary age. And the monster lizards which so exuberantly sprang into existence during the middle secondary epoch, had all disappeared when these terrestrials occupied the stage. Wisdom we can trace in all the arrangements; care and goodness are everywhere apparent. The seas swarmed with marine animals, while the terrestrials could scarcely have subsisted on an upheaving earth and new forming land. Quadrupeds roamed not over fields so diversified by the lakes and slimy lagoons in which the Saurians found their convenient habitation. And beyond the simple fact, that Divine will so ordained that such things should be, both in the animal and mineral changes in the history of our planet, we have only a ray of light to guide us in interpreting the revolutions and destructions which are therein so indelibly recorded.

The mollusca and shell families appear and depart along with the calcareous deposits which inclose their remains; but we know as much of the source of the one as of the range and limit of, or the causes which destroyed, the other. Orthoceræ and nautili have survived all changes, and have maintained in the types by which they are represented their old instincts and predaceous propensities. The holoptychii and dipteri perished, just as the materials of the new red sandstone were being deposited, and whose identity, in all essential mineral qualities, differs in nothing from the old red in which they are entombed. The flora of the carboniferous age came and went with the suddenness and entireness of an eastern dynasty, the gorgeous spoils of which are all that remain to attest its former greatness. The mammoths, dinotheriums, and kindred pachyderms of the tertiary groups had all left the earth on the dawn of the human epoch. And now, since the commencement of that epoch, we find that entire families have become extirpated, that species of others have been driven from their former localities, and that generally, both of vegetables and animals, the geographical distribution is being,

year by year, greatly modified. During the last century, the introduction into Germany of some new species of insects, and their multiplication, utterly destroyed forests of vast extent; and every year, in some quarter of the globe, we hear of equally mighty catastrophes produced by equally minute insidious causes. The organic things of earth, it would thus appear, have their terms of existence of longer and shorter duration, and the race at last dies out equally with the individuals which compose it. No better reason for this can be assigned, than that such is, and has always been, the course of nature. Particular families of plants and animals are cotemporaneous with particular groups of rocks with these they are observed for the first time; at the close of the deposit, all farther traces of their remains are lost; and, in so far, there is ground for arguing that the same general causes were concerned in effecting the successive changes, organic as well as inorganic, of the periods and formations in question. What these causes were, it may never be permitted to science fully to determine. It was indeed, the opinion of Cuvier, that in the mammoth epoch a change of climate effected the destruction of this giant family of pachyderms. This change of climate has been accounted for by Murchison and others, especially in Siberia, where so many remains are found, by an elevation of the country to the height of one or two hundred feet above its former level. And doubtless, by such a change, animal as well as vegetable life must, in many specific forms, have been greatly affected.

66

There can be little doubt, however, of the most perfect adaptive arrangements prevailing through all the geologal epochs, some of which have been plausibly conjectured. As reptiles, for instance, differ from birds and mammals, in having a lower and simpler structure of the lungs and heart, and therefore a less active performance of the respiratory functions, they become less dependent on the atmosphere or oxygen for existence. "Hence," says Professor Owen, "from their extraordinary prevalence in the secondary periods, under varied modifications of size and structure, severally adapting them to the performance of those tasks in the economy of organic nature which are now assigned to the warm-blooded and quick-breathing classes, the physiologist is led to conjecture that the atmosphere had not undergone those

ges, which the consolidation and concentration of certain of elements in subsequent additions to the earth's crust may e occasioned during the long lapse of ages during which the nction of so large a proportion of the reptilian class took place. d if the chemist, by wide and extended views of his science in ation to geology, and mineralogy, should demonstrate, as the anist from considerations of the peculiar features of the extinct ra has been led to suspect, that the atmosphere of this globe merly contained more carbon and less oxygen than at present, en the anatomist might, à priori, have concluded that the highest asses of animals suited to the respiration of such a medium must ve been the cold-blooded fishes and reptiles. And beside, the obability of such a condition of the zoological series being conected with the chemical modifications of the air, the terrestrial ptiles, from the inferior energy of their muscular contractions, nd still more from the greater irritability of the fibers and ower of continuing their actions, would constitute the highest rganized species, best adapted to exist under greater atmopheric pressure than operates on the surface of the earth at the resent time.”

By parity of reasoning it may be inferred, that as great changes would be effected in the waters of the globe as in the constituents of the atmosphere; and, while thus preparing for the introduction of new families of animals, the destruction of already existing tribes may be as conclusively imagined. The various calcareous deposits in the mountain limestone, magnesian, oolite, and chalk periods, would imply very different qualities in the condition of the ocean; an infusion or abstraction of ingredients as favorable to the existence of one kind of animal life as they would be destructive of another. A period of great plutonic action, too, when vast masses of melted matter, charged with metallic and other substances, were poured over the bed of the sea, could not fail to have considerable influence upon many of the inhabitants of the deep; and while providence was making arrangements for an increase, or diversity, or for higher types of animal life, the existence of other forms and classes was ordained to terminate.

The introduction of new and higher races upon the earth has

thus been accounted for. "Through such a medium as the air," says the authority quoted above, “approaching in a corresponding degree to the physical properties of water, a cold-blooded animal might even rise above the surface, and wing its heavy flight, since this would demand less energetic muscular actices than are now requisite for such a kind of locomotion: and thus w may conceive why the atmosphere of our planet, during the earlier oolite periods, may have been traversed by creatures of no higher organization than saurians. If we may presume to conjecture that atmospheric pressure has been diminished, by a change in the composition, as well as by a diminution of the general mass of the air, the beautiful adaptation of the structure of birds to a medium thus rendered both lighter and more invigorating, by the abstraction of carbon and an increase of oxygen, must be appreciable by every physiologist. And it is not without interest to observe, that the period when such change would be thus indicated by the first appearance of birds in the Wealden strata, is likewise characterized by the prevalence of those dinosaurian reptiles, which in structure most nearly approach mammalia, and which in all probability, from their correspondence with crocodiles in the anatomy of the thorax, enjoyed a circulation as complete as that of the crocodile when breathing freely on dry land."

Again, it is conjectured—“The first indications of the warmblooded classes would appear, if introduced into the reptilian era, under the form of such small insectivorous mammals as are known at the present day to have a lower amount of respiration than the rest of the class; and the earliest discovered remains of mammalia,—as, for example, those in the Stonesfield oolite,—are actually the jaws of such species, with which are combined the characters of that order, Marsupialia, which is most nearly related to the oviparous vertebrata.”

It has been seen that igneous and aqueous agents have remodeled, from time to time, the physical geography of the globe. Can it admit of a doubt that changes in the physical structure of the earth's surface will be accompanied with other changes in the organic productions of extensive areas? Species, it is well known, both of plants and animals, are limited to particular localities of