evolved from the lower tribes in the ancient Europeo-Asiatic continent. The defenceless lower Marsupials are worsted in the "struggle for existence" that ensues. The higher "tooth and claw exterminate the lower races in the Palearctic region; but in Australia the isolated, these Marsupials, free from the irruption of later carnivores with tooth and claw, and protected by the intervening sea from the inroad of the higher quadruped-races, flourish and grow. As time passes, the original species of Marsupials-that is, the first emigrants to Australia-vary, and, through variation, produce new races and species of these quadrupeds. Australia in due time developes a quadruped population of its own, which repeats the varied features of mammalian existence elsewhere. Thus again there is presented to our view an illustration of the double work of land alteration and specific or biological change, in developing a strange and wondrous population on the surface of the earth.

Last of all, the history of the opossums and their distribution, now limited to the New World, falls under the sway of the same efficient explanation, supported by every fact of life and by all the details of geological science.

Commencing their existence in the Palearctic region-their fossil remains occurring, for example, in the Eocene rocks of France-the opossums represent a race which never at any period of their existence have dwelt in Australian territory. Their occurrence in America is explicable, not on any theory of possible connection between America and Australia, but on the plain hypothesis of their migration to the New World by a continuous land-surface in the middle or towards the end of the Tertiary period, from Europe or from Northern Asia as a centre. Their earliest fossils, in the New World, occur in the American Post-Pliocene-that is, long after their first appearance in European formations. Passing thus to the New World, the opossums migrated southwards, where they flourished and grew apace, comparatively unmolested by carnivora or other enemies. Again extending their range northwards, they are found in North America; and they thus represent in the Western Hemisphere a flourishing remnant of a race killed off from the Old World, and driven, by stress of outward circumstances, to seek refuge in the New.

Not less interesting is it to find that the existing life of Australia at large fully endorses the biological dictum that in this island territory we find still represented the life which was once world-wide in its extent in the Triassic and Oolitic period, in which period Australia severed its connection with the Asiatic continent. As the Marsupial

quadrupeds of the Oolite overran the existing land area of that day, so they flourish, and flourish alone, in the Australia we ourselves know. As the spine-bearing Port Jackson shark swims in the Australian seas to-day, so the spiny fishes Acrodus and Strophodus swam in the Oolitic seas that washed Palearctic and other coasts. As the shell-fish Trigonia lived in the seas of the Stonesfield Slate period around our shores, so that Trigonia still persists on the Australian coasts alone. And, lastly, as the Araucarian pines and cycads grew in Oolitic times in our own area, so they grow now in Australian territory a remnant, like the quadrupeds and fishes, of a flora and fauna once well-nigh universal, but now limited to the region of the earth wherein alone the original conditions of their life are truly represented.

If geological change isolating or uniting land areas, and the variation and modification of species consequent upon such separation or union, be thus credited with constituting the great factors and powers which have produced the existing distribution of animals and plants, and which have regulated that distribution in all time past, we may now briefly glance at the main features which the great biological regions of the world have exhibited in relation to the changes and alterations of their boundaries they have from time to time undergone.

Whilst the late Sir Charles Lyell and other geologists were found not so long ago to declare that the great continents of the world "shift their positions entirely in the course of ages," a clearer understanding of geological evidence has completely established the doctrine of the permanence of the great continental areas, and of the general stability in time of the main masses of the land. It is needful to make ourselves acquainted with this fact, inasmuch as, if the distribution of life depends primarily on the distribution of land and sea, a clear understanding of the agencies regulating the development of animals and plants on the globe will be gained only when the physical changes in question are duly appreciated. The geological evidence, then, goes to prove that, whilst the general mass of the continents has remained unchanged, their minor features and more intimate details have been subjected to frequent disturbance. Thus in the past, as at present, the uniformity of geological action postulates the work of rivers in eroding the land, of the sea in defacing the coasts, of ice in carving the land surface, and of volcanic action in depressing this area or elevating that, and in causing the sea to flow here, or to repress its march there. Professor A. Geikie maintains that the stratified rocks, instead of being formed in the beds of deep

oceans, "have all been deposited in comparatively shallow water." And, again, this eminent geologist remarks of the manner in which this earth's materials have been formed, that "From all this evidence, we may legitimately conclude that the present land of the globe, though formed in great measure of marine formations, has never lain under the deep sea; but that its site must always have been near land. Even its thick marine limestones,” adds Professor Geikie," are the deposits of comparatively shallow water."

Thus with the proofs of the general permanence and stability of our great continents at hand, we can completely account for all the plainer facts, and for many of the difficulties, of distribution. For example, we infer that about the middle of the Tertiary period, Europe and Asia, as at present, formed one continuous land surface, which contained as its inhabitants the elephants, rhinoceroses, giraffes, apes, and other forms now found only in the Oriental and Ethiopian regions. Antelopes were then found in Southern Europe, and the giraffes extended from the South of Europe to the North of India. But we must likewise take account of those more intimate changes of land and sea which accompanied the general permanence of the continents. At the time we are considering, Africa south of the desert was a large island; India and Ceylon were isolated by sea from Asia; Northern Africa was united to the South of Europe; Asia Minor was joined to Greece ;-the outlines of the great zoological regions of the Old World were, in short, actually mapped out. in the middle of the Tertiary period in the then existing lands and seas. But neither the detached India nor the isolated Africa possessed the abundant quadruped life of Europe and Asia. They possessed only the lower life of the Eocene time. When, however, the next series of physical alterations took place, when land passages arose between Europe and Asia together on the one hand, and Africa and India on the other, the higher quadrupeds migrated to these areas. There some adapted themselves to their new conditions, and flourished in their new localities, whilst others succumbed to the more rigorous surroundings which faced them. The antelopes, for instance, migrating to Africa, flourished in Ethiopia, because there they found a plentiful vegetation and the other conditions of life calculated to produce the development of new species by the modification of the old. The bears and deer are unknown in Africa, on the contrary, since they were later comers in European territory, and because they found migration a difficult or impossible task. The fauna which Europe then gave to Ethiopia was killed off in the former by the climatal changes which succeeded these

Miocene times, and which left the region to be peopled after the glacial cold, by the hardier forms which we now call our representative animals. Similarly, India as the Oriental province possessed when detached from Asia its own lower Edentates, and its lemurs; but when it became united with the Asiatic continent, it received from the north, like Africa, its new complement of animals-its monkeys, tigers, elephants, and other forms-these animals arising in the ancient Palearctic land, whence, as we have seen, the earlier marsupials themselves migrated to people the other quarters of the globe.

The history of the New World is equally instructive, both as regards the proofs it supplies of the permanence of the continents, along with the evidences of the same laws of dispersal and migration of life which the consideration of the Old World areas affords. The first fact of importance in the scientific history of the New World areas consists in the knowledge that in the Post-Pliocene times the life of the Nearctic region approached very closely to that of the Palearctic province. In the Post-Pliocene formations of America, we find the fossil remains of numerous carnivora, horses, camels, bisons, and elephants. Of the living elephants, as we have seen, the existing New World knows nothing. The horses were reintroduced by man; whilst the buffalo certainly represents the bisons, and the llamas similarly represent the camels. Before the Post-Pliocene time, geology reveals that America possessed rhinoceroses, special forms of ruminants, and a porcupine decidedly of Old World type. In the still earlier Miocene period, North America had its lemurs— now limited to India, Africa, and Madagascar-many carnivora, camels, deer, and tapirs. Earlier still, that is, in the Eocene period, there lived in North America animals unlike any forms now existent. There were the Tillodonts and Dinoceratidæ of Professor Marsh, which appear to have united in themselves the characters of several distinct orders of quadrupeds. There is thus every reason to believe that in the Post-Pliocene period, at least, and in earlier times likewise, there was free land communication between the Palearctic and Nearctic areas. So that it requires no stretch of hypothesis to assume that the horses, camels, elephants, and other quadrupeds of America-proved to be near allies of European fossil forms—must have freely intermingled with those of Europe. That Europe, or, more properly, the Palearctic region, must have been the original source whence the Nearctic land obtained its mastodons, porcupines, deer, and other quadrupeds, is proved by the fact that these animals are known to have lived and flourished in Europe long YOL. CCLIII. NO. 1821. A A

before they occurred in America. So that, as Mr. Wallace puts it, "As the theory of evolution does not admit the independent development of the same group in two disconnected regions to be possible, we are forced to conclude that these animals have migrated from one continent to the other. Camels, and perhaps ancestral horses," adds Mr. Wallace, "on the other hand, were more abundant and more ancient in America, and may have migrated thence into Northern Asia." The physical difficulties of such a land connection at Behring's Straits or across Baffin's Bay, are not, it may be remarked, by any means insuperable.

Then, likewise, we must take into account the share which South America, or the Neotropical region, has had in influencing the distribution of life in the New World at large. North America seems in the Post-Pliocene epoch to have been a literal focus wherein Palearctic life commingled with life from the South. Thus the North American Post-Pliocene deposits give us sloths and other forms of Edentate mammals, llamas, tapirs, and peccaries, all of which are typically South American; whilst some are identical with living Neotropical species. The bone-caves of South America show us that this region, like Australia, possessed in Post-Pliocene times the same description of quadruped life that now distinguishes it. As giant kangaroos lived in Australia, so gigantic sloths and armadillos lived in South America; and its chinchillas, spiny rats, bats, and peculiar monkeys were likewise existent then as now. In addition, we find that, as North America possessed its peculiar groups of lower quadrupeds in its tillodonts and other forms, so South America likewise had its special types of life, such as the Macrauchenia, resembling the tapirs, and the Toxodonts, related at once to the hoofed quadrupeds and to other groups. But, whilst the quadruped immigrations into North America likewise affected South America, it must be borne in mind that the isolation and separation of South America from the northern part of the continent, as indicated by its regional distinctness, must have largely influenced the development of its own peculiar life-just, indeed, as the peculiarities of North America are due to its separation, in turn, from the Palearctic. And when we further discover the all-important fact that the fishes on each side of the Isthmus of Panama are identical, the theory of the relatively recent continuity of sea at this point, and the consequent separation of Neotropical from Nearctic land, rises into the domain of fact. Thus we see in North America a region which has repeatedly received and exchanged tenants with the great Europeo-Asiatic continent; which has, in consequence, developed a

area.