Before speculating further on this subject, it will be well to lay before our readers a summary of South American palæontology, after which we shall be in a better position to draw correct inferences from the whole body of the evidence.
South America.
Unfortunately, our knowledge of the interesting fossil fauna of this continent, is almost wholly confined to the Post-Pliocene and Pliocene periods. A few remains have been discovered in deposits believed to be of Eocene age, but nothing whatever representing the vast intervening period, so rich in peculiar forms of animal life both in North America and Europe.
Fauna of the Brazilian caves.—What we know of the Post-Pliocene period is chiefly due to the long-continued researches of Dr. Lund in the caves of Central Brazil, mostly situated in a district near the head waters of the San Francisco river in the Province of Minas Geraes. The caves are formed in limestone rocks, and are so numerous that Dr. Lund visited thousands, but only sixty contained bones in any quantity. These caves have a floor of reddish earth, often crowded with bones. In one experiment, half a cubic foot of this earth contained jaws of 400 opossums, 2,000 mice, besides remains of bats, porcupines and small birds. In another trial, the whole of the earth in a cavern was carried out for examination, amounting to 6,552 firkins; and, from a calculation made by measured samples, it was estimated to contain nearly seven millions of jaw-bones of cavies, opossums, porcupines, and mice, besides small birds, lizards, and frogs. This immense accumulation is believed to have been formed from the bodies of animals brought into the cavern by owls; and, as these are unsocial birds, the quantity found implies animmense lapse of time, probably some thousands of years. More than 100 species of Mammalia, in all, were obtained in these caves. Some were living species or closely allied to such; but the majority were extinct, and a considerable number, about one-fourth, belonged to extinct genera, or genera not now inhabiting South America. Stone implements and human remains were found in several of the caves with extinct animals. The following enumeration of these remains is from the corrected list of M. Gervais.
Primates.—Extinct species ofCebus,Callithrix, andJacchus—South American genera of monkeys; with an extinct genus,Protopithecus—an animal of large size but belonging to the American family Cebidæ.
Chiroptera.—Species belonging to the South American Phyllostomidæ, and to two South American genera of other families.
Carnivora.—Five species ofFelis, some allied to living animals, others extinct; a species of the widespread extinct genusMachairodus; and a small species referred toCynælurus, the genus containing the hunting leopard now found only in Africa and India. Canidæ are represented byCanisandIcticyon(a living Brazilian species of the latter genus), and the extinct genusSpeothos. Mustelidæ are represented by extinct species of the South American generaMephitisandGalictis. Procyonidæ, by a species ofNasua. Ursidæ, byArctotherium, a genus closely resembling, if not identical with, that containing the "spectacled bear" of Chili.
Ungulata.—Equus,Tapirus,Dicotyles,Auchenia,Cervus,Leptotherium, andAntilope, are the cave-genera of this order.EquusandAntelopeare particularly interesting, as representing groups forming no part of existing South American zoology; while the presence also ofLeptotherium, an extinct genus of antelopes, shows that the group was fairly represented in South America at this comparatively recent period.
Proboscidea.—A species ofMastodon, found also in the Pliocene of La Plata, represents this order.
Rodentia.—These abound.Dasyprocta,Cælogenys,Cavia,Kerodon, all living genera of Caviidæ, are represented byextinct species.Cercolabes, the 'tree porcupine' (Cercolabidæ) has two species, one as large as a peccary;Myopotamus,Loncheres,Carterodon, are existing genera of spiny rats (Echimyidæ); and there are two extinct genera of the same family,LonchophorusandPhyllomys.Lagostomus(Chinchillidæ), the viscacha of the Pampas, is represented by an extinct species. There is also an extinct species ofLepus; several species ofHesperomysandOxymycterus; and a largeArvicola, a genus not living in South America.
Edentata.—These, which constitute the great feature of the existing South American fauna, were still more abundant and varied in the Cave period, and it is remarkable that most of them are extinctgenera. The armadillos are alone represented by living forms,Dasypus, andXenurus;EurydonandHeterodon, are extinct genera of the same family, as well asChlamydotherium—huge armadillos the size of a tapir or rhinoceros, andPachytherium, which was nearly as large. The ant-eaters are represented only byGlossotherium, an extinct form allied toMyrmecophagaandManis. The sloths were more numerous, being represented by the extinct generaCælodon,SphenodonandOchotherium, the last of large size. The huge terrestrial sloths—Megatheridæ, also abounded; there being species ofMegatheriumandMegalonyx, as well as the alliedScelidotherium, supposed to have some affinity for the AfricanOrycteropus.
Marsupials.—No new forms of these appear, but numerous species ofDidelphys, all closely allied to opossums still living in South America.
The preceding sketch of the wonderful cave fauna of Central Brazil, is sufficient to show that it represents, in the main, a period of great antiquity. Not only are almost the whole of the species extinct, but there are twenty extinct genera, and three others not now inhabitants of South America. The fact that so few remains of the living animals of the country are found in these caves, indicates that some change of physical conditions has occurred since they were the receptacles of so many of the larger animals; and the presence of many extinct genera oflarge size, especially among the Edentata and American families of Rodents, are additional proofs of a very high antiquity. Yet many of these cave animals are closely allied to those which are found in North America in the Post-Pliocene deposits only, so that we have no reason to suppose the cave-fauna to be of much earlier date. But the great amount of organic change it implies, must give us an enlarged idea of the vast periods of time, as measured by years, which are included in this, the most recent of all geological epochs.
Pliocene Period of Temperate South America.—We have now to consider the numerous remains of extinct animals found in various deposits in the Pampas, and in Patagonia, and a few in Bolivia. The age of these is uncertain; but as they are very similar to the cave-fauna, though containing a somewhat larger proportion of extinct genera and some very remarkable new forms, they cannot beverymuch older, and are perhaps best referred at present to the newer portion of the Pliocene formation.
Carnivora.—The genusMachairodusor sabre-toothed tigers, represents the Felidæ. There are several species of wolves (Canis); a weasel (Mustela); two bears of the Brazilian cave-genusArctotherium; and the extinct European genusHyænarctos.
Ungulata.—There are two species ofEquus, found in the Pampas, Chili, and Bolivia; two ofMacrauchenia, an extraordinary extinct group allied to the tapir andPalæotherium, but with the long neck, and general size of a camel. A second species found on the highlands of Bolivia is much smaller.
A more recent discovery, in Patagonia, is the almost perfect series of teeth of a large animal namedHomalodontotherium; and which is believed by Professor Flower, who has described it, to have been allied toRhinoceros, and still more to the MioceneHyracodonfrom North America; and also to present some resemblances toMacrauchenia, and though much more remotely, to the curious genusNesodonmentioned further on.
The Artiodactyla, or even-toed Ungulates, are represented by a species ofDicotyles, or peccary, found in the deposits of thePampas; byAuchenia, or llama, of which three extinct species inhabited Bolivia, in which country two allied but extinct genera,PalæolamaandCamelotherium, have also been found. Three species of deer (Cervus), from the Pampas deposits, complete the list of Pliocene Ungulates.
Proboscidea.—The cave species ofMastodonis found also in the Pampas deposits, and another in the Andes of Chili and Bolivia.
Rodents.—These are not so numerous as in the caves. There are species of the existing genera,KerodonandCavia(Caviidæ);Lagostomus(Chinchillidæ);Ctenomys(Octodontidæ);Lepus(hare);HesperomysandOxymycterus(Muridæ);Arvicola, a genus not living in South America; and an extinct genus,Cardiodus. There is also a remarkable extinct form,Typotherium, larger than the capybara, and having affinities to Edentates and Ungulates. Three species have been found in the Pampas deposits.
Edentata.—These are as abundant and remarkable as in the cave deposits.Scelidotherium,Megatherium,Megalonyx,GlossotheriumandDasypus, have already been noticed as from the Brazilian caves. We have here, in addition, the hugeMylodonallied to theMegatherium, and the allied genera—GnathopsisandLestodon. We then come to the huge extinct armadillos,GlyptodonandSchistopleurum, the former consisting of numerous species, some of which were as large as an elephant. Another genus,Eutatus, is allied to the livingthree-bandedarmadillos; and a species of the existing genusEuphractushas been found in Bolivia.
Toxodontidæ.—There remain a number of huge animals rivalling the Megatherium in size, and forming the generaToxodonandNesodon, but whose position is doubtful. Several species have been found in the deposits of the Pampas and Patagonia. They are allied at once to Ungulates, Rodents, Edentates, and the aquatic Sirenia, in so puzzling a manner that it is impossible to determine to what order they belong, or whether they require a new order to be formed for their reception. Some are believed to date back to the Miocene period, and they indicate what strange forms may still be discovered, should anyproductive deposits be found in South America of middle Tertiary age.
Pliocene Mammalia of the Antilles.—These may be noticed here, as they are of special interest, proving the connection of the larger West Indian Islands with the Continent some time in the later Tertiary period. They consist of remains of two large animals belonging to the South American Chinchillidæ, found in cave deposits in the island of Anguilla, and forming two new genera,AmblyrhizaandLoxomylus; and remain allied toMegalonyxfrom Cuba, which have been namedMegalocnusandMyomorphus.
Eocene fauna of South America.—The few remains yet discovered in the Tertiary deposits of the Pampas which are believed to be of Eocene age, are exceedingly interesting, because they show us another change in the scenery of the great drama of life; there being apparently a considerable resemblance, at this epoch, between South America and Europe. They consist of a large extinct feline animal,Eutemnodus; ofPalæotheriumandAnoplotherium, the well-known extinct Ungulates of the European Tertiaries, and which have never been found in North America; and of three genera of Rodents,—Theridromys, allied toEchimys, and found also in the Eocene and Miocene of France;Megamys, allied to the livingCapromysof the Antilles, and also toPalæomys, an extinct form of the French Miocene; and a very large animal referred toArvicola, a genus found also in the Pliocene deposits of South America, and abundant in the northern hemisphere. No Edentates have been found.
The resemblances of this fauna to that of Europe rather than to any part of America, are so strong, that they can hardly be accidental. We greatly want, however, more information on this point, as well as some corresponding evidences as to the condition of West and South Africa about the same epoch, before we can venture to speculate on their bearing as regards the early migrations of organic forms.
General Remarks on the Extinct Mammalian Fauna of the Oldand New Worlds.—Leaving the more special applications of palæontological evidence to be made after discussing the relations of the existing fauna of the several regions, we propose here to indicate briefly, some of the more general deductions from the evidence which has now been laid before our readers.
The first, and perhaps the most startling fact brought out by our systematic review, is the very recent and almost universal change that has taken place in the character of the fauna, over all the areas we have been considering; a change which seems to be altogether unprecedented in the past history of the same countries as revealed by the geological record. In Europe, in North America, and in South America, we have evidence that a very similar change occurred about the same time. In all three we find, in the most recent deposits—cave-earths, peat-bogs, and gravels—the remains of a whole series of large animals, which have since become wholly extinct or only survive in far-distant lands. In Europe, the great Irish elk, theMachairodusand cave-lion, the rhinoceros, hippopotamus, and elephant;—in North America, equally large felines, horses and tapirs larger than any now living, a llama as large as a camel, great mastodons and elephants, and abundance of huge megatheroid animals of almost equal size;—in South America these same megatheroids in greater variety, numerous huge armadillos, a mastodon, large horses and tapirs, large porcupines, two forms of antelope, numerous bears and felines, including aMachairodus, and a large monkey,—have all become extinct since the deposition of the most recent of the fossil-bearing strata. This is certainly not a great while ago, geologically; and it isalmostcertain that this great organic revolution, implying physical changes of such vast proportions that they must have been due to causes of adequate intensity and proportionate range, has taken place since man lived on the earth. This is proved to have been the case in Europe, and is supported by much evidence both as regards North and South America.
It is clear that so complete and sudden a change in the higher forms of life, does not represent the normal state of things. Species and genera have not, at all times, become so rapidly extinct. The time occupied by the "Recent period," that is thetimesincethese changes took place is, geologically, minute. The time of the whole of the Post-Pliocene period, as measured by the amount of physical andgeneralorganic change known to have taken place, is exceedingly small when compared with the duration of the Pliocene period, and still smaller, probably, as compared with the Miocene. Yet during these two periods we meet with no such break in the continuity of the forms of life, no such radical change in thecharacterof the fauna (though the number of specific and generic changes may be as great) as we find in passing from the Post-Pliocene to recent times. For example, in Central Europe numerous hyænas, rhinoceroses, and antelopes, with the greatMachairodus, continued from Miocene all through Pliocene into Post-Pliocene times; while hippopotami and elephants continued to live through a good part of the Pliocene and Post-Pliocene periods,—and then all suddenly became extinct or left the country. In North America there has been more movement of the fauna in all the periods; but we have similar great felines, horses, mastodons, and elephants, in the Pliocene and Post-Pliocene periods, whileRhinocerosis common to the Miocene and Pliocene, and camels range continuously from Miocene, through Pliocene, to Post-Pliocene times;—when all alike became extinct. Even in South America the evidence is, as far as it goes, all the same way. We findMachairodus,Equus,Mastodon,Megatherium,Scelidotherium,Megalonyx, and numerous gigantic armadillos, alike in the caves and in the stratified tertiary deposits of the Pampas;—yet all have since passed away.
It is clear, therefore, that we are now in an altogether exceptional period of the earth's history. We live in a zoologically impoverished world, from which all the hugest, and fiercest, and strangest forms have recently disappeared; and it is, no doubt, a much better world for us now they have gone. Yet it is surely a marvellous fact, and one that has hardly been sufficiently dwelt upon, this sudden dying out of so many large mammalia, not in one place only but over half the land surface of the globe. We cannot but believe that there must have been some physical cause for this great change; and it must have been a cause capable of acting almost simultaneously over largeportions of the earth's surface, and one which, as far as the Tertiary period at least is concerned, was of an exceptional character. Such a cause exists in the great and recent physical change known as "the Glacial epoch." We have proof in both Europe and North America, that just about the time these large animals were disappearing, all the northern parts of these continents were wrapped in a mantle of ice; and we have every reason to believe that the presence of this large quantity of ice (known to have been thousands of feet if not some miles in thickness) must have acted in various ways to have produced alterations of level of the ocean as well as vast local floods, which would have combined with the excessive cold to destroy animal life. There is great difference of opinion among geologists and physicists as to the extent, nature, and duration of the Glacial epoch. Some believe it to have prevailed alternately in the northern and southern hemispheres; others that it was simultaneous in both. Some think there was a succession of cold periods, each lasting many thousands of years, but with intercalated warm periods of equal duration; others deny that there is any evidence of such changes, and maintain that the Glacial epoch was one continuous period of arctic conditions in the temperate zones, with some fluctuations perhaps but with no regular alternations of warm periods. Some believe in a huge ice-cap covering the whole northern hemisphere from the pole to near 50° north latitude in the eastern, and 40° in the western hemisphere; while others impute the observed effects either to glaciers from local centres, or to floating icebergs of vast size passing over the surface during a period of submersion.
Without venturing to decide which of these various theories will be ultimately proved to be correct, we may state, that there is an increasing belief among geologists in the long duration of this ice-period, and the vast extent and great thickness attained by the ice-sheet. One of the most recent, and not the least able, of the writers on this question (Mr. Belt) shows strong reasons for adopting the view that the ice-period was simultaneous in both hemispheres; and he calculates that the vast amount of water abstracted from the ocean and locked upin mountains of ice around the two poles, would lower the general level of the ocean about 2,000 feet. This would be equivalent to a general elevation of the land to the same amount, and would thus tend to intensify the cold; and this elevation may enable us to understand the recent discoveries of signs of glacial action at moderate elevations in Central America and Brazil, far within the tropics. At the same time, the weight of ice piled up in the north would cause the land surface to sink there, perhaps unequally, according to the varying nature of the interior crust of the earth; and since the weight has been removed land would rise again, still somewhat irregularly; and thus the phenomena of raised beds of arctic shells in temperate latitudes, are explained.
Now, it is evident, that the phenomena we have been considering—of the recent changes of the mammalian fauna in Europe, North America, South Temperate America, and the highlands of Brazil—are such as might be explained by the most extreme views as to the extent and vastness of the ice-sheet, and especially as to its simultaneous occurrence in the northern and southern hemispheres; and where two such completely independent sets of facts are found to combine harmoniously, and supplement each other on a particular hypothesis, the evidence in favour of that hypothesis is greatly strengthened. An objection that will occur to zoologists, may here be noticed. If the Glacial epoch extended over so much of the temperate and even parts of the tropical zone, and led to the extinction of so many forms of life even within the tropics, how is it that so much of the purely tropical fauna of South America has maintained itself, and that there are still such a vast number of forms, both of mammalia, birds, reptiles, and insects, that seem organized for an exclusive existence in tropical forests? Now Mr. Belt's theory, of the subsidence of the ocean to the extent of about 2,000 feet, supplies an answer to this objection; for we should thus have a tract of lowland of an average width of some hundreds of miles, added to the whole east coast of Central and South America. This tract would, no doubt, become covered with forests as it was slowly formed, would enjoy a perfectlytropical climate, and would thus afford an ample area for the continued existence and development of the typical South American fauna; even had glaciers descended in places so low as what is now the level of the sea, which, however, there is no reason to believe they ever did. It is probable too, that this low tract, which all round the Gulf of Mexico would be of considerable width, offered that passage for intermigration between North and South America, which led to the sudden appearance in the former country in Post-Pliocene times, of the huge Megatheroids from the latter; a migration which took place in opposite directions as we shall presently show.
The birth-place and migrations of some mammalian families and genera.—We have now to consider a few of those cases in which the evidence already at our command, is sufficiently definite and complete, to enable us to pronounce with some confidence as to the last movements of several important groups of mammalia.
Primates.—The occurrence in North America of numerous forms of Lemuroidea, forming two extinct families, which are believed by American palæontologists to present generalized features of both Lemuridæ and Hapalidæ, while in Europe only Lemurine forms allied to those of Africa have occurred in deposits of the same age (Eocene), renders it possible that the Primates may have originated in America, and sent one branch to South America to form the Hapalidæ and Cebidæ, and another to the Old World, giving rise to the lemurs and true apes. But the fact that apes of a high degree of organization occur in the European Miocene, while in the Eocene, a monkey believed to have relations to the Lemuroids and Cebidæ has also been discovered, make it more probable that the ancestral forms of this order originated in the Old World at a still earlier period. The absence of any early tertiary remains from the tropical parts of the two hemispheres, renders it impossible to arrive at any definite conclusions as to the origin of groups which were, no doubt, always best developed in tropical regions.
Carnivora.—This is a very ancient and wide-spread group, the families and genera of which had an extensive range in veryearly times. The true bears (Ursus) are almost the only important genus that seems to have recently migrated. In Europe it dates back to the Older Pliocene, while in North America it is Post-Pliocene only. Bears, therefore, seem to have passed into America from the Palæarctic region in the latter part of the Pliocene period. They probably came in on the north-west, and passed down the Andes into South America, where one isolated species still exists.
Ungulata.—Horses are very interesting. In Europe they date back under various forms to the Miocene period, and trueEquusto the Older Pliocene. In North America they are chiefly Pliocene, trueEquusbeing Post-Pliocene, with perhaps one or two species Newer Pliocene; but numerous ancestral forms date back to the Miocene and Eocene, giving a more perfect "pedigree of the horse" than the European forms, and going back to a more primitive type—Orohippus. In South America,Equusis the only genus, and is Post-Pliocene or at most Newer Pliocene. While, therefore, the ancient progenitors of the Equidæ were common to North America and Europe, in Miocene and even Eocene times, true horses appear to have arisen in the Palæarctic region, to have passed into North America in the latter part of the Pliocene period, and thence to have spread over all suitable districts in South America. They were not, however, able to maintain themselves permanently in their new territory, and all became extinct; while in their birth-place, the Old World, they continue to exist under several varied forms.
True tapirs are an Old World group. They go back to the Lower Miocene in Europe, while in both North and South America they are exclusively Post-Pliocene. They occur in France down to the Newer Pliocene, and must, about that time, have entered America. The land connection by which this and so many other animals passed between the Old and New Worlds in late Tertiary times, was almost certainly in the North Pacific, south of Behring's Straits, where, as will be seen by our general map, there is a large expanse of shallow water, which a moderate elevation would convert into dry land, in a sufficiently temperate latitude.
The peccary (Dicotyles), now a characteristic South American genus, is a recent immigrant from North America, where it appears to have been developed from ancestral forms of swine dating back to the Miocene period.
Antelopes are an Old World type, but a few of them appear to have entered North, and reached South America in late Pliocene times. Camels, strange to say, are a special North American type, since they abounded in that continent under various ancient forms in the Miocene period. Towards the end of that period they appear to have entered eastern Asia, and developed into the SiberianMerycotheriumand the North IndianCamelus, while in the Pliocene age the ancestral llamas entered South America.
Cervidæare a wide-spread northern type in their generalized form, but true deer (Cervus) are Palæarctic. They abounded in Europe in Miocene times, but only appear in North and South America in the later Pliocene and Post-Pliocene periods.
True oxen (Bovinæ) seem to be an Oriental type (Miocene), while they appear in Europe only late in the Pliocene period, and in America are confined to the Post-Pliocene.
Elephants (Elephantidæ) are an Old World type, abounding in the Miocene period in Europe and India, and first appearing in America in Post-Pliocene or later Pliocene times. Ancestral forms, doubtfully Proboscidean (Dinocerata), existed in North America in the Eocene period, but these became extinct without leaving any direct descendants, unless theBrontotheridæand rhinoceroses may be so considered.
Marsupials are almost certainly a recent introduction into South and North America from Asia. They existed in Europe in Eocene and Miocene times, and presumably over a considerable part of the Old World; but no trace of them appears in North or South America before the Post-Pliocene period.
Edentata.—These offer a most curious and difficult problem. In South America they abound, and were so much more numerous and varied in the Post-Pliocene and Pliocene, that we may be sure they lived also in the preceding Miocene period. A few living Edentates are scattered over Africa and Asia, andthey flourished in Europe during the Miocene age—animals as large (in some species) as a rhinoceros, and most allied to living African forms. In North America no trace of Edentata has been found earlier than the Post-Pliocene period, or perhaps the Newer Pliocene on the west coast. Neither is there any trace of them in South America in the Eocene formations; but this may well be owing to our very imperfect knowledge of the forms of that epoch. Their absence from North America is, however, probably real; and we have to account for their presence in the Old World and in South America. Their antiquity is no doubt very great, and the point of divergence of the Old World and South American groups, may take us back to early Eocene, or even to Pre-Eocene times. The distribution of land and sea may then have been very different from what it is now; and to those who would create a continent to account for the migrations of a beetle, nothing would seem more probable than that a South Atlantic continent, then united parts of what are now Africa and South America. There is, however, so much evidence for the general permanence of what are now the great continents and deep oceans, that Professor Huxley's supposition of a considerable extension of land round the borders of the North Pacific Ocean in Mesozoic times, best indicates the probable area in which the Edentate type originated, and thence spread over much of the Old World and South America. But while in the latter country it flourished and increased with little check, in the other great continents it was soon overcome by the competition of higher forms, only leaving a few small-sized representatives in Africa and Asia.
VARIOUS EXTINCT ANIMALS;—AND ON THE ANTIQUITY OF THE GENERA OF INSECTS AND LAND MOLLUSCA.
EXTINCT MAMMALIA OF AUSTRALIA.
These have all been obtained from caves and late Tertiary or Post-Tertiary deposits, and consist of a large number of extinct forms, some of gigantic size, but all marsupials and allied to the existing fauna. There are numerous forms of kangaroos, some larger than any living species; and among these are two genera,ProtemnodonandSthenurus, which Professor Garrod has lately shown to have been allied, not to any Australian forms, but to theDendrolagior tree-kangaroos of New Guinea. We have also remains ofThylacinusandDasyurus, which now only exist in Tasmania; and extinct species ofHypsiprymnusandPhascolomys, the latter as large as a tapir. Among the more remarkable extinct genera areDiprotodon, a huge thick-limbed animal allied to the kangaroos, but nearly as large as an elephant;Nototherium, having characters ofMacropusandPhascolarctoscombined, and as large as a rhinoceros; andThylacoleo, a phalanger-like marsupial nearly as large as a lion, and supposed by Professor Owen to have been of carnivorous habits, though this opinion is not held by other naturalists.
Here then we find the same phenomena as in the other countries we have already discussed,—the very recent disappearance of a large number of peculiar forms, many of them far surpassing in size any that continue to exist. It hardly seems probable that in this case their disappearance can have been due to the direct effects of the Glacial epoch, since no very extensiveglaciation could have occurred in a country like Australia; but if the ocean sank 2,000 feet, the great eastern mountain range might have given rise to local glaciers. It is, however, almost certain that during late Tertiary times Australia must have been much more extensive than it is now. This is necessary to allow of the development of its peculiar and extensive fauna, especially as we see that that fauna comprised animals rivalling in bulk those of the great continents. It is further indicated by the relations with New Guinea, already alluded to, and by the general character of the various faunas which compose the Australian region, details of which will be found in the succeeding part of this work. The lowering of the ocean during the Glacial period would be favourable to the still further development of the fauna of such a country; and it is to the unfavourable conditions produced by its subsequent rising—equivalent to a depression of the land to the amount of two thousand feet—that we must impute the extinction of so many remarkable groups of animals. It is not improbable, that the disappearance of the ice and the consequent (apparent) subsidence of the land, might have been rapid as compared with the rate at which large animals can become modified to meet new conditions. Extensive tracts of fertile land might have been submerged, and the consequent crowding of large numbers of species and individuals on limited areas would have led to a struggle for existence in which the less adapted and less easily modifiable, not the physically weaker, would succumb.
There is, however, another cause for the extinction of large rather than small animals whenever an important change of conditions occurs, which has been suggested to me by a correspondent,[4]but which has not, I believe, been adduced by Mr. Darwin or by any other writer on the subject. It is dependent on the fact, that large animals as compared with small ones are almost invariably slow breeders, and as they also necessarily exist in much smaller numbers in a given area, they offer far less materials for favourable variations than do smaller animals. In such an extreme case as that of the rabbit and elephant, theyoung born each year in the world are probably as some millions to one; and it is very easily conceivable that in a thousand years the former might, under pressure of rapidly changing conditions, become modified into a distinct species, while the latter, not offering enough favourable variations to effect a suitable adaptation, would become extinct. We must also remember the extreme specialization of many of the large animals that have become extinct—a specialization which would necessarily render modification in any new direction difficult, since the inherited tendency of variation would probably be to increase the specialization in the same directions which had heretofore been beneficial. If to these two causes we add the difficulty of obtaining sufficient food for such large animals, and perhaps the injurious effects of changes of climate, we shall not find it difficult to understand how such a vast physical revolution as the Glacial epoch, with its attendant phenomena of elevations and subsidences, icy winds, and sudden floods by the bursting of lake barriers, might have led to the total extinction of a vast number of the most bulky forms of mammalia, while the less bulky were able to survive, either by greater hardiness of constitution or by becoming more or less modified. The result is apparent in the comparatively small or moderate size of the species constituting the temperate fauna, in all parts of the globe.
It is much to be regretted that no mammalian remains of earlier date have been found in Australia, as we should then see if it is really the case that marsupials have always formed its highest type of mammalian life. At present its fossil fauna is chiefly interesting to the zoologist, but throws little light on the past relations of this isolated country with other parts of the globe.
Mammalian remains in the Secondary Formations.
In the oldest Tertiary beds of Europe and North America, we have (even with our present imperfect record) a rich and varied mammalian fauna. As compared with our living or recent highly specialized forms, it may be said to consist of generalised types; but as compared with any primeval mammalian type, it must be pronounced highly specialised. Not only are such diversifiedgroups as Carnivora, Perrissodactyle and Artiodactyle Ungulates, Primates, Chiroptera, Rodents, and Marsupials already well marked, but in many of these there is a differentiation into numerous families and genera of diverse character. It is impossible therefore to doubt, that many peculiar forms of mammalia must have lived long anterior to the Eocene period; but there is unfortunately a great gap in the record between the Eocene and Cretaceous beds, and these latter being for the most part marine continue the gap as regards mammals over an enormous lapse of time. Yet far beyond both these chasms in the Upper Oolitic strata, remains of small mammalia have been found; again, in the Stonesfield slate, a member of the Lower Oolite, other forms appear. Then comes the marine Lias formation with another huge gap; but beyond this again in the Upper Trias, the oldest of the secondary formations, mammalian teeth have been discovered in both England and Germany, and these are, as nearly as can be ascertained, of the same age as theDromatheriumalready noticed, from North America. They have been namedMicrolestes, and show some resemblance to those of the West AustralianMyrmecobius. In the Oolitic strata numerous small jawbones have been found, which have served to characterise eight genera, all of which are believed to have been Marsupials, and in some of them a resemblance can be traced to some of the smaller living Australian species. These, however, are mere indications of the number of mammalia that must have lived in the secondary period, so long thought to be exclusively "the age of reptiles;" and the fact that the few yet found are at all comparable with such specialised forms as still exist, must convince us, that we shall have to seek far beyond even the earliest of these remains, for the first appearance of the mammalian type of vertebrata.
Extinct Birds.
Compared with those of mammalia, the remains of birds are exceedingly scarce in Europe and America; and from the wandering habits of so many of this class, they are of much less valueas indications of past changes in physical geography. A large proportion of the remains belong to aquatic or wading types, and as these have now often a world-wide range, the occurrence of extinct forms can have little bearing on our present inquiry. There are, however, a few interesting cases of extinct land-birds belonging to groups now quite strangers to the country in which they are found; and others scarcely less interesting, in which groups now peculiar to certain areas are shown to have been preceded by allied species or genera of gigantic size.
Palæarctic Region and N. India.—In the caves and other Post-Pliocene deposits of these countries, the remains of birds almost all belong to genera now inhabiting the same districts. Almost the only exceptions are, the great auk and the capercailzie, already mentioned as being found in the Danish mounds; the latter bird, withTetrao albus, in Italian caverns; and a species of pheasant (Phasianus) said to have occurred in the Post-Pliocene of France, considerably west of the existing range of the genus in a wild state.
In the preceding Pliocene deposits, but few remains have been found, and all of existing genera but one, a gallinaceous bird (Gallus bravardi) allied to the domestic fowl and peacock.
The Miocene beds of France and Central Europe have produced many more remains of birds, but these, too, are mostly of existing European genera, though there are some notable exceptions. Along with forms undistinguishable from crows (Corvus), shrikes (Lanius), wagtails (Motacilla), and woodpeckers (Picus), are found remains allied to the Oriental edible-nest swift (Collocalia) andTrogon; a parrot resembling the African genusPsittacus; an extinct formNecrornis, perhaps allied to the plantain-eaters (Musophaga);Homalophus, doubtfully allied to woodpeckers, andLimnatornisto the hoopoes. The gallinaceous birds are represented by three species of pheasants, some very close to the domesticated species;Palæoperdixallied to the partridges; andPalæortyx, small birds allied to the American genusOrtyx, but with larger wings. There are also species ofPteroclesallied to living birds, and a small pigeon. There are numerous living genera of Accipitres; such as eagle (Aquila),kite (Milvus), eagle-owl (Bubo), and screech-owl (Strix); with the African secretary-bird (Serpentarius), and some extinct forms, asPalæocercus,PalæohierixandPalæetus.
Aquatic and wading birds were abundant, including numerous rails, bustards, herons, sandpipers, gulls, divers, and pelicans. There were also many ducks, some allied to the genusDendrocygna; the Oriental genus of storks,Leptoptilus;Ibidipodia, a remarkable form allied toIbisandCiconia;Elornis, nearLimosa;Pelagornis, a large bird allied to gannets and pelicans;Hydrornis, allied to the ducks and petrels;Dolichopterus, allied to plovers. Perhaps the most interesting of these extinct birds are, however, the flamingoes, represented by forms hardly distinguishable from living species, and by one extinct genusPalælodus, which had very long toes, and probably walked on aquatic plants like the tropical jacanas.
The Miocene beds of North India have furnished few birds; the only one of geographical interest being an extinct species of ostrich, not very different from that now inhabiting Arabia.
On the whole, the birds of Europe at this period were very like those now living, with the addition of a few tropical forms. These latter were, however, perhaps more numerous and important than they appear to be, as they belong to inland and forest-haunting types, which would not be so frequently preserved as the marsh and lake-dwelling species. Taking this into consideration, the assemblage of Miocene birds accords well with what we know of the mammalian fauna. We have the same indications of a luxuriant vegetation and subtropical climate, and the same appearance of Oriental and especially of African types.Trogonis perhaps the most interesting of all the forms yet discovered, since it furnishes us with a central point whence the living trogons of Asia, Africa, and South America might have diverged.
In the Eocene we find ourselves almost wholly among extinct forms of birds. The earliest known Passerine bird is here met with, inProtornis, somewhat similar to a lark, found in the Lower Eocene of Switzerland; while another Passerine form,Palægithalus, and one allied to the nuthatch (Sitta), have beendiscovered in the Upper Eocene of Paris. Picariæ of equal antiquity are found.Cryptornis, from the Paris Eocene, andHalcyornisfrom the Lower Eocene of the Isle of Sheppey, were both allied to kingfishers; while a form allied toCentropusa genus of cuckoos, or, as Milne-Edwards thinks, to the MadagascarLeptosomus, has been found in the Upper Eocene of France. SeveralAccipitresof somewhat doubtful affinities have been found in the same country; whileLithornis, from the Lower Eocene of the Isle of Sheppey, was a small vulturine bird supposed to be allied to the American group,Cathartes. Among the waders, some extinct forms of plovers have been found, and a genus (Agnopterus), allied to the flamingoes; while there are many swimming birds, such as pelicans, divers, and several extinct types of doubtful affinities. Most intersting of all is a portion of a cranium discovered in the Lower Eocene of Sheppey, and lately pronounced by Professor Owen to belong to a large Struthious bird, allied to the New ZealandDinornisand also perhaps to the ostrich. Another gigantic bird is theGastornis, from the Lower Eocene of Paris, which was as large as an ostrich, but which is believed to have been a generalised type, allied to wading and swimming birds as well as to the Struthiones.
Beyond this epoch we have no remains of birds in European strata till we come to the wonderfulArchæopteryxfrom the Upper Oolite of Bavaria; a bird of a totally new type, with a bony tail, longer than the body, each vertebra of which carried a pair of diverging feathers.
North America.—A number of bird-remains have lately been found in the rich Tertiary and Cretaceous deposits of the United States; but here, too, comparatively few are terrestrial forms. No Passerine bird has yet been found. The Picariæ are represented byUintornis, an extinct form allied to woodpeckers, from the Eocene of Wyoming. Species of turkey (Meleagris) occur in the Post-Pliocene and as far back as the Miocene strata, showing that this interesting type is a true denizen of temperate North America. The other birds are,Accipitres; waders and aquatics of existing genera; and a number of extinct forms of the two latter orders—such as,Aletornisan Eocene wader;Palæotringa, allied to the sandpipers, andTelmatobiusto the rails, both Cretaceous; withGraculavus, allied toGraculus;Laornisallied to the swans;Hesperornisa gigantic diver; andIcthyornisa very low form, with biconcave vertebra, such as are only found in fishes and some reptiles—also from Cretaceous deposits.
South America.—The caverns of Brazil produced thirty-four species of birds, most of them referable to Brazilian genera, and many to still existing species. The most interesting were two species of American ostrich (Rhea), one larger than either of the living species; a large turkey-buzzard (Cathartes); a new species of the very isolated South American genusOpisthocomus; and aCariama, or allied new genus.
Madagascar and the Mascarene Islands.—We have here only evidence of birds that have become extinct in the historical period or very little earlier. First we have a group of birds incapable of flight, allied to pigeons, but forming a separate family,Dididæ; and which, so far as we yet know, inhabited Mauritius, Rodriguez, and probably Bourbon.Aphanapteryx, an extinct genus of rails, inhabited Mauritius; and another genus, (Erythromachus), Rodriguez. A large parrot, said by Prof. Milne Edwards to be allied toAraandMicroglossus, also inhabited Mauritius; and another allied toEclectus, the island of Rodriguez. None of these have been found in Madagascar; but a gigantic Struthious bird,Æpyornis, forming a peculiar family distinct both from the ostriches of Africa and theDinornisof New Zealand inhabited that island; and there is reason to believe that this may have lived less than 200 years ago.
New Zealand.—A number of extinct Struthious birds, forming two families,DinornithidæandPalapterygidæ, have been found in New Zealand. Some were of gigantic size. They seem allied both to the livingApteryxof New Zealand and the emu of Australia. They are quite recent, and some of them have probably lived within the last few centuries. Remains ofDinornishave also been found in a Post-Pliocene deposit in Queensland, N. E. Australia[5]—a very important discovery, as itgives support to the theory of a great eastward extension of Australia in Tertiary times.
Extinct Tertiary Reptiles.
These will not occupy us long, as no very great number are known, and most of them belong to a few principal forms of comparatively little geographical interest.
Tortoises are perhaps the most abundant of the Tertiary reptiles. They are numerous in the Eocene and Miocene formations both in Europe and North America. The generaEmysandTrionyxabound in both countries, as well as in the Miocene of India. Land tortoises occur in the Eocene of North America and in the Miocene of Europe and India, where the hugeColossochelys, twelve feet long, has been found. In the Pliocene deposits of Switzerland the living American genusChelydrahas been met with. These facts, together with the occurrence of a livingspeciesin the Miocene of India, show that this order of reptiles is of great antiquity, and that most of the genera once had a wider range than now.
Crocodiles, allied to the three forms now characteristic of India, Africa, and America, have been found in the Eocene of our own country, and several species ofCrocodilushave occurred in beds of the same age in North America.
Lizards are very ancient, many small terrestrial forms occurring in all the Tertiary deposits. A species of the genusChamæleois recorded from the Eocene of North America, together with several extinct genera.
Snakes were well developed in the Eocene period, where remains of several have been found which must have been from twelve to twenty feet long. An extinct species of true viper has occurred in the Miocene of France, and one of the Pythonidæ in the Miocene brown coal of Germany.
Batrachia occur but sparingly in a fossil state in the Tertiary deposits. The most remarkable is the large Salamander (Andreas) from the Upper Miocene of Switzerland, whichis allied to theMenopomaliving in North America. Species of frog (Rana), andPalæophryusan extinct genus of toads, have been found in the Miocene deposits of Germany and Switzerland.
Fresh water fish are almost unknown in the Tertiary deposits of Europe, although most of the families and some genera of living marine fish are represented from the Eocene downwards.
Antiquity of the Genera of Insects.
Fossil insects are far too rarely found, to aid us in our determination of difficult questions of geographical distribution; but in discussing these questions it will be important to know, whether we are to look upon the existing generic forms of insects as of great or small antiquity, compared with the higher vertebrates; and to decide this question the materials at our command are ample.
The conditions requisite for the preservation of insects in a fossil state are no doubt very local and peculiar; the result being, that it is only at long intervals in the geological record that we meet with remains of insects in a recognisable condition. None appear to have been found in the Pliocene formation; but in the Upper Miocene of Œninghen in Switzerland, associated with the wonderfully rich fossil flora, are found immense quantities of insects. Prof. Heer examined more than 5,000 specimens belonging to over 800 species, and many have been found in other localities in Switzerland; so that more than 1,300 species of Miocene insects have now been determined. Most of the orders are represented, but the beetles (Coleoptera) are far the most abundant. Almost all belong to existing genera, and the majority of these genera now inhabit Europe, only three or four being exclusively Indian, African, or American.
In the Lower Miocene of Croatia there is another rich deposit of insects, somewhat more tropical in character, comprising large white-ants and dragon-flies differently marked from anynow inhabiting Europe. A butterfly is also well preserved, with all the markings of the wings; and it seems to be aJunonia, a tropical genus, though it may be aVanessa, which is European, but the fossil most resembles Indian species ofJunonia.
The Eocene formations seem to have produced no insect remains; but they occur again in the Upper Cretaceous at Aix-la-Chapelle, where two butterflies have been found,Cyllo sepultaandSatyrites Reynesii, both belonging to the Satyridæ, and the former to a genus now spread over Africa, India, and Australia.
A little earlier, in the Wealden formation of our own country, numerous insects have been found, principally dragon flies (Libellula,Æshna); aquatic Hemiptera (Velia Hydrometra); crickets, cockroaches, and cicadas, of familiar types.
Further back in the Upper Oolite of Bavaria—which produced the wonderful long-tailed bird,Archæopteryx—insects of all orders have been found, including a moth referred to the existing genusSphinx.
In the Lower Oolite of Oxfordshire many fossil beetles have been found whose affinities are shown by their names:—Buprestidium,Curculionidium,Blapsidium,Melolonthidium, andPrionidium; a wing of a butterfly has also been found, allied to the Brassolidæ now confined to tropical America, and namedPalæontina oolitica.
Still more remote are the insects of the Lias of Gloucestershire, yet they too can be referred to well-known family types—Carabidæ, Melolonthidæ, Telephoridæ, Elateridæ, and Curculionidæ, among beetles; Gryllidæ and Blattidæ among Orthoptera; withLibellula,Agrion,Æshna,Ephemera, and some extinct genera. When we consider that almost the only vertebrata of this period were huge Saurian reptiles like theIcthyosaurus,Plesiosaurus, andDinosaurus, with the flying Pterodactyles; and that the great mass of our existing genera, and even families, of fish and reptiles had almost certainly not come into existence, we see at once that types of insect-form are, proportionately, far more ancient. At this remote epoch we find the chief family types (thegeneraof the time of Linnæus) perfectly differentiatedand recognisable. It is only when we go further back still, into the Palæozoic formations, that the insect forms begin to show that generalization of type which renders it impossible to classify them in any existing groups. Yet even in the coal formation of Nova Scotia and Durham, the fossil insects are said by competent entomologists to be "allied toEphemera," "nearBlatta," "nearPhasmidæ;" and in deposits of the same age at Saarbrück near Trèves, a well-preserved wing of a grasshopper or locust has been found, as well as a beetle referred to the Scarabeidæ. More remarkable, however, is the recent discovery in the carboniferous shales of Belgium, of the clearly-defined wing of a large moth (Breyeria borinensis), closely resembling some of the Saturniidæ; so that we have now all the chief orders of Insects—including those supposed to be the most highly developed and the most recent—well represented at this very remote epoch. Even the oldest insects, from the Devonian rocks of North America, can mostly be classed as Neuroptera or Myriapoda, but appear to form new families.
We may consider it, therefore, as proved, that many of the larger and more important genera of insects date back to the beginning of the Tertiary period, or perhaps beyond it; but the family types are far older, and must have been differentiated very early in the Secondary period, while some of them perhaps go back to Palæozoic times. The great comparative antiquity of thegenerais however the important fact for us, and we shall have occasion often to refer to it, in endeavouring to ascertain the true bearing of the facts of insect distribution, as elucidating or invalidating the conclusions arrived at from a study of the distribution of the higher animals.
Antiquity of the Genera of Land and Fresh-Water Shells.
The remains of land and fresh-water shells are not much more frequent than those of insects. Like them, too, their forms are very stable, continuing unchanged through several geologicalperiods. In the Pliocene and Miocene formations, most of the shells are very similar to living species, and some are quite identical. In the Eocene we meet with ordinary forms of the generaHelix,Clausilia,Pupa,Bulimus,Glandina,Cyclostoma,Megalostoma,Planorbis,PaludinaandLimnæa, some resembling European species, others more like tropical forms. A British Eocene species ofHelixis still living in Texas; and in the South of France are found species of the Brazilian sub-generaMegaspiraandAnastoma. In the secondary formation no true land shells have been found, but fresh water shells are tolerably abundant, and almost all are still of living forms. In the Wealden (Lower Cretaceous) and Purbeck (Upper Oolite) are foundUnio,Melania,Paludina,Planorbis, andLimnæa; while the last named genus occurs even in the Lias.
The notion that land shells were really not in existence during the secondary period is, however, proved to be erroneous by the startling discovery, in the Palæozoic coal measures of Nova Scotia, of two species of Helicidæ, both of living genera—Pupa vetusta, andZonites priscus. They have been found in the hollow trunk of aSigillaria, and in great quantities in a bed full of Stigmarian rootlets. The most minute examination detects no important differences of form or of microscopic structure, between these shells and living species of the same genera! These mollusca were the contemporaries of Labyrinthodonts and strange Ganoid fishes, which formed almost the whole vertebrate fauna. This unexpected discovery renders it almost certain, that numbers of other existing genera, of which we have found no traces, lived with these two through the whole secondary period; and we are thus obliged to assume as a probability, that any particular genus has lived through a long succession of geological ages. In estimating the importance of any peculiarities or anomalies in the geographical distribution of land shells as compared with the higher vertebrates, we shall, therefore, have to keep this possible, and even probable high antiquity, constantly in mind.
We have now concluded our sketch of Tertiary Palæontology as a preparation for the intelligent study of the GeographicalDistribution of Land Animals; and however imperfectly the task has been performed, the reader will at all events have been convinced that some such preliminary investigation is an essential and most important part of our work. So much of palæontology is at present tentative and conjectural, that in combining the information derived from numerous writers, many errors of detail must have been made. The main conclusions have, however, been drawn from as large a basis of facts as possible; and although fresh discoveries may show that our views as to the past history of some of the less important genera or families are erroneous, they can hardly invalidate our results to any important degree, either as regards the intercommunications between separate regions in the various geological epochs, or as to the centres from which some of the more important groups have been dispersed.
PART III.
ZOOLOGICAL GEOGRAPHY:
A REVIEW OF THE CHIEF FORMS OF ANIMAL LIFE IN THE SEVERAL REGIONS AND SUB-REGIONS, WITH THE INDICATIONS THEY AFFORD OF GEOGRAPHICAL MUTATIONS.