THE DISTRIBUTION OF ANIMALS AND PLANTS AS RELATED TO GEOGRAPHICAL AND GEOLOGICAL CHANGES.

All are now agreed that to explain the extraordinary and often apparently anomalous distribution of animals and plants over the surface of the earth, and the occurrence of like forms in very distant localities, and even on islands separated by vast stretches of ocean from one another and from the continents, we must invoke the aid of geology. We must have reference to those changes of climate and of elevation which have occurred in the more recent periods of the earth's history, and must carry with us the idea, at first not apparently very reasonable, that living beings have existed much longer than many of the lands which they inhabit, or at least than the present state of those lands in reference to isolation or continental connection. To what extent we may further require to call in the aid of varietal or specific modification to explain the facts, may be more doubtful; and I think we shall find that a larger acquaintance with geological truths would enable us to dispense with the aid of hypotheses of evolution, at least in so far as the local establishment of new generic and specific types is concerned.

One of the most remarkable and startling results of geological investigation, and one which must be accepted as an established fact, independently of all theoretical explanations, is that the earth has experienced enormous revolutions of climate within comparatively late periods, and since the date of the introduction of many existing species of animals and plants. To this great truth, in some of its bearings, I have endeavoured to direct attention in the previous articles. In the present case it will be necessary to consider these vicissitudes in their more general aspects, and with some reference to their effects on the distribution of living beings.

The modern or human period of geology, that in which man and his contemporaries are certainly known to have inhabited the earth, was immediately preceded by an age of climatal refrigeration known as the Glacial or Ice age. This was further characterized not only by a prevalence of cold, unexampled so far as known either before or since, but by immense changes of the relative levels of sea and land, amounting, in some cases, at least, to several thousands of feet. The occurrence of these changes is clearly proved by the undoubted traces of the action of ice, whether land ice or floating ice, on all parts of our continents, half way to the equator, and by the occurrence of sea terraces and modern marine shells at high levels on mountains and table-lands. Perhaps we scarcely realize as we should the stupendous character of the changes involved in the driftage of heavy ice over our continents as far south as the latitude of 40°, in the deposit of boulders on hills several thousands of feet in height, and in the occurrence of shells of species still living in the sea, in beds raised to more than twelve hundred feet above its present level. Yet such changes must have occurred in the latest geological period immediately preceding that in which we live. Proceeding farther back in geological time, we find the still more extraordinary fact that in the middle and earlier Tertiary the northern hemisphere enjoyed a climate so much more mild than that which now prevails, that plants at present confined to temperate latitudes could flourish in Greenland and Spitzbergen.[183] The age in which we live is thus one of mediocrity, attaining neither to the Arctic rigour of the later Pleistocene, nor to the universal mildness of the preceding Miocene.

The causes of these changes of climate we have discussed elsewhere. It remains for us now to consider the actual condition of our present continents, and the bearing of past conditions on the distribution of their living inhabitants.

In speaking of continents and islands, it may be as well to remark at the outset that all the land existing, or which probably has at any time existed, consists of islands great or small. It is all surrounded by the ocean. Two of the greater masses of land are, however, sufficiently extensive to be regarded as continents, and from their very extent and consequent permanence may be considered as the more special homes of the living beings of the land. Two other portions of land, Australia and the Antarctic polar continent, may be regarded either as smaller continents or large islands, but partake of insular rather than continental characters in their animals and plants. All the other portions of land are properly islands; but while these islands, and more especially those in mid-ocean, cannot be regarded as the original homes of many forms of life, we shall find that they have a special interest as the shelters and refuges of numerous very ancient and now decaying species.

The two great continents of America and Eurasia have been the most permanent portions of the land throughout geological time, some parts of them having always been above water, probably from the Laurentian age downward, though at various times they have been reduced to little more than groups of islands. On them, and more especially in their more northern parts, in which the long continuance of daylight in summer seems in warm periods to have been peculiarly favourable to the introduction of new vegetable and animal forms, and to the giving to them that vigour necessary for active colonization, have originated the greater number of the inhabitants of the land.

Regarded as portions of the earth's crust, the continents are areas in which the lateral thrust, caused by the secular contraction of the interior of the earth and unequal settlement of the crust, has ridged up and folded the rocks, producing mountain chains. This process began in the earliest geological periods, and has been repeated at long intervals, the original lines of folding guiding those formed in each new thrust proceeding from the broad oceanic areas. Along the ridges thus produced, and in the narrower spaces between them, the greater part of the sediment carried by water was laid down, thus producing plateaus in connection with the mountain-chains, while the weight of new sediments and the removal of matter from other areas by denudation, have been constantly producing local depression and elevation. The tendency of the ocean to be thrown toward the poles by the retardation of the earth's rotation, alternating with great collapses of the crust at the equator proceeding from the same cause, along with the secular cooling, have produced alternate submergence and emergence of these plateaus. This has been further complicated by the constant tendency of the Arctic and Antarctic currents, aided by ice, to drift solid materials, set free by the vast denuding action of frost, from the polar to the temperate regions, and by the further tendency of animal life to heap up calcareous accumulations under the warm waters of the tropical regions. All these changes, as already stated, have conspired to modify the directions of the great oceanic currents, and to produce vicissitudes of climate under which animals and plants have been subjected in geological time to those migrations, extinctions, and renovations of which their fossil remains and present distribution afford evidence.

Still, it is true that throughout the whole of these great mutations, since the beginning of geological history, there seems never to have been any time when the ocean so regained its dominion as to produce a total extinction of land life; still less was there any time when the necessary conditions of all the various forms of marine life failed to be found; nor was there any climatal change so extreme as to banish any of the leading forms of life from the earth. To geologists it is not necessary to say that the conclusions sketched above are those that have been reached as the results of long and laborious investigation, and which have been illustrated and established by Lyell, Dana, Wallace,[184] and many other writers.[185] Let us now place beside them some facts as to the present distribution of life, and of the agencies which influence it.

Just as political geography sometimes presents boundaries not in accordance with the physical structure of countries, so the distribution of animals and plants shows many peculiar and unexpected features. Hence naturalists have divided the continents into what Sclater has called zoological regions, which are, so to speak, the great empires of animal life, divisible often by less prominent boundaries into provinces. In vegetable life similar boundaries may be drawn, more or less coincident with the zoological divisions. Zoologically, North America and Greenland may be regarded as one great region, the Nearctic, or new Arctic, the prefix not indicating that the animals are newer than those of the old world, which is by no means the case. South America constitutes another region the Neotropical. If now we turn to the greater Eurasian continent, with its two prolongations to the south in Africa and Australia, we shall find the whole northern portion, from the Atlantic to the Pacific, constituting one vast region of animal life, the Palearctic, which also includes Iceland and a strip across North Africa. Africa itself, with Madagascar, whose allegiance is, however, only partial, constitutes the Ethiopian region. India, Burmah, the south of China, and certain Asiatic islands form the Oriental region. Australia, New Guinea, and the Polynesian islands constitute the Australian region. All of these regions may in a geological point of view be considered as portions of old and permanent continental masses, which, though with movements of elevation and depression, have continued to exist for vast periods. Some of them, however, seem to have enjoyed greater immunity from causes of change than others, and present, accordingly, animals and plants having, geologically speaking, an antique aspect in comparison. In this sense the Australian province may be regarded as the oldest of all in the facies of its animal forms, since creatures exist there of genera and families which have very long ago become extinct everywhere else. Next in age to this should rank the Neotropical or South American region, which, like Australia, presents many low and archaic forms of animal life. The Ethiopian region stands next to it in this, the Oriental and Nearctic next, and last and most modern in its aspect is the great Palearctic region, to which man himself belongs, and the animals and plants of which vindicate their claims to youth by that aggressive and colonizing character already referred to, and which has enabled them to spread themselves widely over the other regions, even independently of the influence of man. On the other hand, the animals and plants of the Australian and South American regions show no such colonizing tendency, and can scarcely maintain themselves against those of other regions when introduced among them. Thus we have at once in these continental regions a great and suggestive example of the connection of geographical and geological distribution, the details of which are of the deepest interest, and have not yet been fully worked out. One great principle is, however, sufficiently established; namely, that the northern regions have been the birthplace of new forms of land life, whence they have extended themselves to the south, while the comparative isolation and equable climate of the South American and Australian regions have enabled them to shelter and retain the old moribund tribes.

Those smaller portions of land separated from the continental masses, the islands properly so called, present, as might be expected, many peculiar features. Wallace divides them into two classes, though he admits that these pass into each other. Continental islands are those in the vicinity of continents. They consist of ancient as well as modern rock formations, and contain animals which indicate a former continental connection. Some of these are separated from the nearest mainland only by shallow seas or straits, and may be assumed to have become islands only in recent geological times. Others are divided from the nearest continent by very deep-water, so that they have probably been longer severed from the mainland. These contain more peculiar assemblages of animals and plants than the islands of the former class. Oceanic islands are more remote from the continents. They consist mostly of rocks belonging to the modern geological periods, and contain no animals of those classes which can migrate only by land. Such islands may be assumed never to have been connected with any continent. The study of the indigenous population of these various classes of islands affords many curious and interesting results, which Wallace has collected with vast industry and care, and which, on the whole, he explains in a judicious manner and in accordance with the facts of geology. When, however, he maintains that evolution of the Darwinian type is "the key to distribution," he departs widely from any basis of scientific fact. This becomes apparent when we consider the following results, which appear everywhere in the discussion of the various insular faunas and floras:—(1) None of these islands, however remote, can be affirmed to have been peopled by the spontaneous evolution of the higher animals or plants from lower forms. Their population is in every case not autochthonous, but derived. (2) Even in those which are most distant from the continents, and may be supposed to have been colonized in very ancient times, there is no evidence of any very important modification of their inhabitants. (3) While the facts point to the origin of most forms of terrestrial life in the Palearctic and Nearctic regions, they afford no information as to the manner or cause of their origination. In short, so far is evolution from being a key to distribution, that the whole question would become much more simple if this element were omitted altogether. A few examples may be useful to illustrate this, as well as the actual explanation of the phenomena afforded by legitimate science.

The Azores are situated in a warm temperate latitude about 900 miles west of Portugal, and separated from it by a sea 2,500 fathoms in depth. The islands themselves are almost wholly volcanic, and the oldest rocks known in them are of late Miocene age. There is no probability that these islands have ever been connected with Europe or Africa, nor is there at present any certainty that they have been joined to one another, or have formed part of any larger insular tract. In these islands there is only one indigenous mammal, a bat, which is identical with a European species, and no doubt reached the islands by flight. There is no indigenous reptile, amphibian, or fresh-water fish. Of birds there are, exclusive of waterfowl, which may be regarded as visitors, twenty-two land birds; but of these, four are regarded as merely accidental stragglers, so that only eighteen are permanent residents. Of these birds fifteen are common European or African species, which must have flown to the islands, or have been drifted thither in storms. Of the remaining three, two are found also in Madeira and the Canaries, and therefore may reasonably be supposed to have been derived from Africa. One only is regarded as peculiar to the Azores, and this is a bullfinch, so nearly related to the European bullfinch that it may be regarded as merely a local variety. Wallace accounts for these facts by supposing that the Azores were depopulated by the cold of the Glacial age, and that all these birds have arrived since that time. There is, however, little probability in such a supposition. He further supposes that fresh supplies of stray birds from the mainland, arriving from time to time, have kept up the identity of the species. Instead of evolution assisting him, he has thus somewhat to strain the facts to agree with that hypothesis. Similar explanations are given for the still more remarkable fact that the land plants of the Azores are almost wholly identical with European and African forms. The insects and the land snails are, however, held to indicate the evolution of a certain number of new specific forms on the islands. The beetles number no less than 212 species, though nearly half of them are supposed to have been introduced by man. Of the whole number 175 are European, 19 are found in Madeira and the Canaries, 3 are American. Fourteen remain to be accounted for, though most of these are closely allied to European and other species; but a few are quite distinct from any elsewhere known. Wallace, however, very truly remarks that our knowledge of the continental beetles is not complete; that the species in question are small and obscure; that they may be survivors of the Glacial period, and may thus represent species now extinct on the mainland; and that for these reasons it may not be irrational to suppose that these peculiar insects either still inhabit, or did once inhabit, some part of the continents, and may be portions of "ancient and widespread groups," once widely diffused, but now restricted to a few insular spots. Among the land snails, if anywhere, we should find evidence either of autochthonous evolution or of specific change. These animals have existed on the earth since the Carboniferous period, and, notwithstanding their proverbial slowness and sedentary habits, they have contrived to colonize every habitable spot of land on the globe—that is, unless in some of these places they have originated de novo. In the Azores there are sixty-nine species of land snails, of which no less than thirty-two, or nearly one-half, are peculiar, though nearly all are closely allied to European types. What, then, is the origin of these thirty-two species, admitting for the sake of argument that they are really distinct, and not merely varietal forms, though it is well known that in this group species are often unduly multiplied. Three suppositions are possible, (1) These snails may have originated in the islands themselves, either by creation or evolution from lower forms; say, from sea snails. (2) They may have been modified from modern continental species. (3) They may be unmodified descendants of species of Miocene or Pliocene age now existing on the continents only as fossils. As the islands appear to have existed since Miocene times, it is no more improbable that species of that or the Pliocene age should have found their way to them than that modern species should; and as we know only a fraction of the Tertiary species of Europe or Africa, it is not likely that we shall be able to identify all of these early visitors. Unfortunately no Miocene or Pliocene deposits holding remains of land snails are known in the Azores themselves, so that this kind of evidence fails us. In Madeira and Porto Santo, however, where there are numerous modern snails, there are Pliocene beds holding remains of these animals. In Madeira there are, according to Lyell, 36 Pliocene species, and in Porto Santo 35, and of these only eight are extinct. Thus we can prove that many of the peculiar species of these islands have remained unchanged since Pliocene times. While differing from modern European shells, several of these species are very near to European Miocene species. Thus we seem to have evidence in the Madeira group, not of modification, but of unchanged survival of Tertiary species long since extinct in Europe. May we not infer that the same was the case in the Azores? These results are certainly very striking when we consider how long the Azores must have existed as islands, how very rarely animals, and especially pairs of animals, must have reached them, and how complete has been the isolation of these animals, and how peculiar the conditions to which they have been subjected in their island retreat.

Other oceanic islands present great varieties of conditions, but leading to similar conclusions. Some, as the Bermudas, seem to have been settled in very modern times with animals and plants nearly all identical with those of neighbouring countries, though even here it would appear that there are some indigenous species which would indicate a greater age or more extended lands, now submerged.[186] Others, like St. Helena, are occupied apparently with old settlers, which may have come to them in early Tertiary, or even in Secondary periods, which have long since become extinct on the continents, and whose nearest analogues are now widely scattered over the world. Islands are therefore places of survival of old species—special preserves for forms of life lost to the continents. One of the most curious of these is Celebes, which seems to be a surviving fragment of Miocene Asia, which, though so near to that continent, has been sufficiently isolated to preserve its old population during all the vast lapse of time between the middle Tertiary and the present period. This is a fact which gives to the oceanic islands the greatest geological interest, and induces us to look into their actual fauna and flora for the representatives of species known on the mainland only as fossils. It is thus that we look to the marsupials of Australia as the nearest analogues of those of the Jurassic of Europe, and that we find in the strange Barramunda (ceratodus) of its rivers the only survivor of a group of fishes once widely distributed, but which has long since perished elsewhere.

Perhaps one of the most interesting examples of this is furnished by the Galapagos Islands, an example the more remarkable that no one who has read in Darwin's fascinating "Journal" the description of these islands, can have failed to perceive that the peculiarities of this strange Archipelago must have been prominent among the facts which first planted in his mind the germ of that theory of the origin of species which has since grown to such gigantic dimensions. It is curious also to reflect that had the bearing of geological history on the facts of distribution been as well known forty years ago as it is now, the reasoning of the great naturalist on this and similar cases might have taken an entirely different direction.

The Galapagos are placed exactly on the equator, and therefore out of reach of even the suspicion of having been visited by the glacial cold, though from their isolation in the ocean, and the effects of the currents flowing along the American coast, their climate is not extremely hot. They are 600 miles west of South America, and the separating ocean is in some parts 3,000 fathoms deep. The largest of the islands is 75 miles in length, and some of the hills attain an elevation of about 4,000 feet, so that there are considerable varieties of station and climate. So far as is known they are wholly volcanic, and they may be regarded as the summits of submerged mountains not unlike in structure to the Andes of the mainland. Their exact geological age is unknown, but there is no improbability in supposing that they may have existed with more or less of extension since the Secondary or Mesozoic period. In any case their fauna is in some respects a survival of that age. Lyell has truly remarked, "In the fauna of the Galapagos Islands we have a state of things very analogous to that of the Secondary period."

Like other oceanic islands, the Galapagos have no indigenous mammals, with the doubtful exception of a South American mouse; but, unlike most others, they are rich in reptiles. At the head of these stand several species of gigantic tortoises. This group of animals, so far as known, commenced its existence in the Eocene Tertiary; and in this and the Miocene period still more gigantic species existed on the continents. It has been supposed that at some such early date they reached the Galapagos from South America. Another group of Galapagan reptiles, perhaps still more remarkable, is that of iguana-like lizards of the genus Amblyrhyncus, which are vegetable feeders,—one of them browsing on marine weeds. They recall the great iguana-like reptiles of the European Wealden, and stand remote from all modern types. There are also snakes of two species, but these are South American forms, and may have drifted to the islands in comparatively recent times on floating trees. The birds are a curious assemblage. A few are common American species, like the rice bird. Others are quaint and peculiar creatures, allied to South American birds, but probably representing forms long since extinct on the continent. The bird fauna, as Wallace remarks, indicates that some of these animals are old residents, others more recent arrivals; and it is probable that they have arrived at various times since the early Tertiary. He assumes that the earlier arrivals have been modified in the islands "into a variety of distinct types"; but the only evidence of this is that some of the species are closely related to each other. It is more likely that they represent to our modern eyes the unmodified descendants of continental birds of the early Tertiary. Darwin remarks that they are remarkably sombre in colouring for equatorial birds; but perhaps their ancestors came from a cooler climate, and have not been able to don a tropical garb; or perhaps they belong to a far-back age, when the vegetable kingdom also was less rich in colouring than it is at present, and the birds were in harmony with it. This, indeed, seems still to be the character of the Galapagos plants, which Darwin says have "a wretched, weedy appearance," without gay flowers, though later visitors have expressed a more favourable opinion.

These plants are in themselves very remarkable, for they are largely peculiar species, and are in many cases confined to particular islands, having apparently been unable to cross from one island to another, though in some way able to reach the group. The explanation is that they resemble North American plants, and came to the Galapagos at a time when a wide strait separated North and South America, allowing the equatorial current to pass through, and drift plants to the Galapagos, where they have been imprisoned ever since. This was probably in Miocene times, and when we know more of the Miocene flora of the southern part of North America we may hope to recover some of the ancestors of the Galapagos plants. In the meantime their probable origin and antiquity, as stated by Wallace, render unnecessary any hypothesis of modification.

Before leaving this subject, it is proper to observe that on the continents themselves there are many remarkable cases of isolation of species, which help us better to understand the conditions of insular areas. The "variable hare" of the Scottish highlands, and of the extreme north of Europe, appears again in the Alps, the Pyrenees, and the Caucasus, being in these mountains separated by a thousand miles of apparently impassable country from its northern haunts. It no doubt extended itself over the intervening plains at a time when Europe was colder than at present. Another curious case is that of the marsh-tit of Europe. This little bird is found throughout south-western Europe. It reappears in China, but is not known anywhere between. In Siberia and northern Europe there is, however, a species or distinct race which connects these isolated patches. In this case, if the Siberian species is truly distinct, we have a remarkable case of isolation and of the permanence of identical characters for a long time; for in that case this bird must be a survivor of the Pliocene or Miocene time. On the other hand, if, as is perhaps more likely, the marsh-tit is only a local variety of the Siberian species, we have an illustration of the local recurrence of this form when the conditions are favourable, even though separated by a great space and long time.

The study of fossils gives us the true meaning of such facts, and causes us to cease to wonder at any case of local repetition of species, however widely separated. The "big trees" of California constitute a remarkable example. There are at present two very distinct species of these trees, both found only in limited areas of the western part of North America. Fossil trees of the same genus (Sequoia) occur as far back as the Cretaceous age; but in this age ten or more species are known. Nor are they confined to America, but occur in various parts of the Eurasian continent as well. Two of the Lower Cretaceous species are so near to the two modern ones that even an unbeliever in evolution may suppose them to be possible ancestors; the remaining eight are distinct, but some of them intermediate in their characters. In the Tertiary period, intervening between the Cretaceous and the modern, fourteen species of Sequoia are believed to have been recognised, and they appear to have existed abundantly all over the northern hemisphere. Thus we know that these remarkable Californian giants are the last remnant of a once widely distributed genus, originating, as far as known, in the Cretaceous age. Now had a grove of Sequoias, however small, survived anywhere in Europe or Asia, and had we no knowledge of the fossil forms, we might have been quite at a loss to account for their peculiar distribution. The fossil remains of the Tertiary rocks, both animal and vegetable, present us with many instances of this kind.

The discussion of the distribution of animals and plants, when carried on in the light of geology, raises many interesting questions as to time, which we have already glanced at, but which deserve a little more attention. As to the vast duration of geological time all geologists are agreed. It is, however, now well understood that science sets certain limits to the time at our disposal. Edward Forbes humorously defined a geologist to be "an amiable enthusiast who is content if allowed to appropriate as much as he pleases of that which other men value least, namely, past time "; but now even the geologist is obliged to be content with a limited quantity of this commodity.

The well-known estimate of Lord Kelvin gave one hundred millions of years as the probable time necessary for the change of the earth from the condition of a molten mass to that which we now see. On this estimate we might fairly have assumed fifty millions of years as covering the time from the Laurentian age to the modern period. The great physicist has, however, after allowing us thus much credit in the bank of time, "suddenly put up the shutters and declared a dividend of less than four shillings in the pound."[187] In other words, he has reduced the time at our disposal to twenty millions of years. Other physicists, reasoning on the constitution of the sun, agree with this latter estimate, and affirm that "twenty millions of years ago the earth was enveloped in the fiery atmosphere of the sun."[188] Geology itself has attempted an independent calculation based on the wearing down of our continents, which appears to proceed at the rate of about a foot in four or five thousand years, and on the time required to deposit the sediments of the several geological formations, estimated at about 70,000 feet in thickness. These calculations would give us, say, eighty-six millions of years since the earth began to have a solid crust, which would, like Lord Kelvin's earlier estimate, give us nearly fifty millions of years for the geological time since the introduction of life. The details of the several estimates made it would be tedious and unprofitable to enter into, but I may state as my own conclusion, that the modern rates of denudation and deposit must be taken as far below the average, and that perhaps the estimate stated by Wallace on data supplied by Houghton, namely, twenty-eight millions, may be not far from the truth, though perhaps admitting of considerable abatement.

This reduced estimate of geological time would still give scope enough for the distribution of animals and plants, but it will scarcely give that required by certain prevalent theories of evolution. When Darwin says, "If the theory (of natural selection) be true, it is indisputable 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 to the present day," he makes a demand which geology cannot supply; for independently of our ignorance of any formations or fossils, except those included in the ArchÆan, to represent this vast succession of life, the time required would push us back into a molten state of the planet. This difficulty is akin to that which meets us with reference to the introduction of many and highly specialized mammals in the Eocene, or of the forests of modern type in the Cretaceous. To account for the origin of these by slow and gradual evolution requires us to push these forms of life so far back into formations which afford no trace of them, but, on the contrary, contain other creatures that appear to be exclusive of them, that our faith in the theory fails. The only theory of evolution which seems to meet this difficulty is that advanced by Mivart, Leconte, and Saporta, of "critical periods," or periods of rapid introduction of new species alternating with others of comparative inaction. This would much better accord with the apparently rapid introduction of many new forms of life over wide regions at the same period. It would also approach somewhat near, in its manner of stating the problem to be solved, to the theory of "creation by law" as held by the Duke of Argyll, or to what may be regarded as "mediate creation," proceeding in a regular and definite manner, but under laws and forces as yet very imperfectly known, throughout geological time.

It seems singular, in view of the facts of palÆontology, that evolutionists of the Darwinian school are so wedded to the idea of one introduction only of each form of life, and its subsequent division by variation into different species, as it progressively spreads itself over the globe, or is subjected to different external conditions. It is evident that a little further and very natural extension of their hypothesis would enable them to get rid of many difficulties of time and space. For example, certain Millipedes and Batrachians are first known in the coal formation, and this not in one locality only, but in different and widely separated regions. If they took beginning in one place, and spread themselves gradually over the world, this must have required a vast lapse of time—more than we can suppose probable. But if, in the coal-formation age, a worm could anywhere change into a Millipede, or a fish into a Batrachian, why might this not have occurred in many places at once? Again, if the oldest known land snails occur in the coal formation, and we find no more specimens till a much later period, why is it necessary to suppose that these creatures existed in the intervening time, and that the later species are the descendants of the earlier? Might not the process have been repeated again and again, so as to give animals of this kind to widely separated areas and successive periods without the slow and precarious methods of continuous evolution and migration? This apparent inconsistency strikes one constantly in the study of discussions of the theory of derivation in connection with geographical and geological distribution. We constantly find the believers in derivation laboriously devising expedients for the migration of animals and plants to the most unlikely places, when it would seem that they might just as well have originated in those places by direct evolution from lower forms. Those who believe in a separate centre of creation for each species must of course invoke all geological and geographical possibilities for the dispersion of animals and plants; but surely the evolutionist, if he has faith in his theory, might take a more easy and obvious method, especially when in any case he is under the necessity of demanding a great lapse of time. That he does not adopt this method perhaps implies a latent suspicion that he must not repeat his miracle too often. He also perceives that if repeated and unlimited evolution of similar forms had actually occurred, there could have remained little specific distinctness, and the present rarity of connecting links would not have occurred. Further, a new difficulty would have sprung up in the geographical and geological relations of species and genera, which would then have assumed too much of the aspect of a preconceived plan. It is only fair to a well-known and somewhat extreme European evolutionist, Karl Vogt, to state that he launches boldly into the ocean of multiple evolution, not fearing to hold that identical species of mollusks have been separately evolved in separate Swiss lakes, and that the horse has been separately evolved in America and in Europe, in the former along a line beginning with Eohippus, and in the latter along an entirely separate line, commencing with PalÆotherium. The serious complications resulting from such admissions are evident, but Vogt deserves credit for faith and consistency beyond those of his teachers.

With reference to the actual distribution of species, the question of time becomes most important when applied to the Glacial period, since it is obvious that much of the present distribution must have been caused, or greatly modified, by that event. The astronomical theory would place the close of the Glacial age as far back as 70,000 or 80,000 years ago. But we have already seen in the chapter on that period that geological facts bring its close to only from 10,000 to 7,000 years before our time. If we adopt the shorter estimates afforded by these facts, it will follow that the submergences and emergences of land in the Glacial ages were more rapid than has hitherto been supposed, and that this would react on our estimate of time by giving facilities for more rapid denudation and deposition. Such results would greatly shorten the duration assignable to the human period. They would render it less remarkable that no new species of animals seem to have been introduced since the Glacial age, that many insular faunas belong to far earlier times, and that no changes even leading to the production of well-marked varieties have occurred in the post-glacial or modern age.

In conclusion, does all this array of fact and reasoning bring us any nearer to the comprehension of that "mystery of mysteries," the origin and succession of life? It certainly does not enable us to point to any species, and to say precisely here, at this time and thus it orginated. If we adopt the theory of evolution, the facts seem to restrict us to that form of it which admits paroxysmal or intermittent introduction of species, depending on the concurrence of conditions favourable to the action of the power, whatever it may be, which produces new organisms. Nor is there anything in the facts of distribution to invalidate the belief in creation, according to definite laws, if that really differs in its nature from certain forms of the hypothesis of evolution. We have also learned that, time being given, animals and plants manifest wonderful powers of migration, that they can vary within considerable limits without ceasing to be practically the same species, and that under certain conditions they can endure far longer in some places than in others. We also see evidence that it is not on limited islands, but on the continents, that land animals and plants have originated, and that swarms of new and vigorous species have issued from the more northern regions in successive periods of favourable Arctic climate. The last of these new swarms or "centres of creation," that with which man himself is more closely connected, belongs to the Palearctic region. We have already seen that in every geological period, when the submerged continental plateaus were pervaded by the warm equatorial waters, multitudes of new marine species appear. In times when, on the contrary, the colder Arctic currents poured over these submerged surfaces, carrying mud and stones, great extinction took place, but certain northern forms of life swarmed abundantly, and when elevation took place, marine species became extinct or were forced to migrate. Everywhere and at all times multiplication of species was promoted by facilities for expansion. The great limestones of our continents, full of corals and shells of new species, belong to times when the ocean spread itself over the continental plateaus, affording wide, untenanted areas of warm and shallow water. The introduction of new faunas and floras on the land belongs to times when vast supplies of food for plants and animals and favourable conditions of existence were afforded by the emergence of new lands possessing fertile soils and abundantly supplied with light, heat, and moisture. Thus geological and geographical facts concur with ordinary observation and experience in reference to varietal forms, in testifying that it is not mere struggle for existence, but facilities for easy existence and rapid extension, that afford the conditions necessary for new and advanced forms of life. These considerations do not, of course, reach to the first cause of the introduction of species, nor even to the precise mode in which this may have acted in any particular case: but perhaps we cannot fully attain to this by any process of inductive inquiry. The study of geographical distribution,' therefore, does not enable us to solve the question of the origin of specific types, but, on the contrary, points to marvellous capacities for migration and a wonderful tenacity of life in species. In these respects, however, it is a study full of interest, and in nothing more so than in the evidence which it affords of the practically infinite provisions made for the peopling of every spot of land or sea with creatures fitted to flourish and enjoy life therein, and to carry on the great and progressive plan of the Creator.

References:—Continental and Island Life, Princeton Review, July, 1881. Address to American Association, 1883. Papers and Addresses to Natural History Society, Canadian Naturalist, Montreal. "The Story of the Earth and Man," 1st ed., 1873, 9th ed., London, 1887.