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In birds we meet with corresponding phenomena. The most abundant and characteristic families of the Old World tropics are replaced here by a series of families of a lower grade of organisation, among which are such remarkable groups as the chatterers (Cotingida), the manakins (Pipridx), the ant-thrushes (Formicariida), the toucans (Rhamphastidae), the motmots (Momotidae), and the humming-birds (Trochilidae), the last perhaps the most remarkable and beautiful of all developments of the bird-type. Parrots are numerous, but these, too, are mostly of peculiar families; while pheasants and grouse are replaced by curassows and tinamous, and there are an unusual number of remarkable and isolated forms of waders.
Reptiles, amphibia, fresh-water fishes, insects, and land-shells, are all equally peculiar and abundant; so that South America presents, on the whole, an assemblage of curious and beautiful natural objects, unsurpassedperhaps even unequalled-in any other part of the globe.
Past Ilistory of the American Continents.—We will now proceed to examine what is known of the past history of the two American continents, and endeavour to determine what have been their former relations to cach other and to the Old World, and how their existing zoological and geographical features have been brought about. And first let us see what knowledge we possess of the past relations of North America with the Eastern continents.
If we go back to that recent period termed the PostPliocenc—corresponding nearly to the Post-Glacial period and to that of pre-historic man in Europe-we
find at once a nearer approximation than now exists between the learctic and Palæarctic faunas. North America then possessed several large cats, six distinct species of the horse family, a camel, two bisons, and four species of elephants and mastodons. A little earlier, in the Pliocene period (although fossil remains of this age are scanty), we have in addition the genus Rhinoceros several distinct camels, some new forms of ruminants and an Old-World form of porcupine. Further back, in the Miocene period, we find a Lemuroid animal, numerous insectivora, a host of carnivora, chiefly feline and canine, a variety of equine and tapirine forms, rhinoceroses, camels, deer, and an extensive extinct family—the Orcodontidae---allied to deer, camels, and swine. There are, however, no elephants. In the still carlier Eocene period most of the animals were peculiar, and unlike anything now living, but some were identical with European types of the same age, as Lophotherium and the family Anchitheridæ.
These facts compel us to believe that at distinct epochs during the Tertiary period the interchange of large mammalia between North America and the Old World has been far more easy than it is now. In the Post-Pliocene period, for example, the horses, elephants, and camels of North America and Europe were so closely allied that their common ancestors must have passed from one continent to the other, just as we feel assured that the common ancestors of the American and European bison, elk, and beaver, must have so migrated. We have further evidence in the curious fact that certain groups appear to come into existence in the one continent much later than in the other. Thus cats, deer, masto
dons, true horses, porcupines, and beavers, existed in Europe long before they appeared in America; and as the theory of evolution does not admit the independent development of the same group in two disconnected regions to be possible, we are forced to conclude that these animals have migrated from one continent to the other. Camels, and perhaps ancestral horses, on the other hand, were more abundant and more ancient in America, and may have migrated thence into Northern Asia.
There are two probable routes for such migrations. From Norway to Greenland by way of Iceland and across Baffin Bay to Irctic America, there is everywhere a comparatively shallow sea, and it is not improbable that during the liocene period, or subsequently, a land communication may have existed here. On the other side of the continent, at Behring Straits, the probability is greater. For here we have a considerable extent of far shallower sea, which a very slight elevation would convert into a broad isthmus connecting North America and North-East Asia. It is true that clephants, horses, deer, and camels would, under existing climatal conditions, hardly range as far north as Greenland and Alaska ; but we must remember that most mysterious yet indisputable fact of the luxuriant vegetation, including even magnolias and other large-leaved evergreens, which flourished in these latitudes during the Miocene period; so that we have all the conditions of favourable climate and abundant food, which would render such interchange of the animals of the two continents not only possible, but inevitable, whenever a land communication was effected ; and there is reason
to believe that this favourable condition of things continued in a diminished degree during a portion of the succeeding Pliocene period.
We must not forget, however, that the faunas of the two continents were always to a great extent distinct and contrasted—such important Old-World groups as the civets, hyenas, giraffes, and liippopotami, never passing to America, while the extinct Oreodlontida, Brontotherida, and many others are equally unknown in the Old World. This renders it probable that the communication even in the north was never of long continuance; while it wholly negatives the theory of an Atlantis bridging over the Atlantic Ocean in the Temperate Zone at any time during the whole Tertiary period.
But the past history of the North-American fauna is complicated by another set of migrations from South America, which, like those from the Old World, appear to have occurred it distant intervals, and to have continued for limited periods. In the Post-Pliocene epoch, along with clephants and horses from Europe or Asia wc, find a host of huge sloths and other Elentata, as well as 1:mas, capybaras, tapirs, and peccaries, all characteristic of South America. Some of these were identical with living species, while others are closely allied to those found fossil in Brazilian caves and other deposits of about the same age, while nothing like them inhabited the Old World at the same period. We are therefore quite sure that they came from some part of the Neotropical region ; but the singular fact is, that in the preceding Pliocenc epoch none of them are found in North America. We conclude, therefore, that their
migration took place at the end of the Pliocene or beginning of the Post-Pliocene cpoch, owing to some specially favourable conditions, but that they rapidly disappeared, having left no survivors. We must, how. cver, study the past history of South America in order to ascertain how far it has been isolated from or connected with the northern continent.
Abundant remains of the Post-Pliocene cpoch from Brazilian caves show us that the fauna of South America which immediately preceded that now existing hed the same general characteristics, but was much richer in large mammalia and probably in many other forms of life. Edentata formed the most prominent feature; but instead of the existing sloths, armadillos, and ant-caters, there were an immense variety of these animals, some of living genera, others altogether different, and many of them of enormous size. There were armadillos as large as the rhinoceros, while the megatherium and several other genera of extinct sloths were of clephantine bulk. The peculiar families of South American rodents -cavies, spiny-rats, and chinchillas—were represented by other species and genera, some of large size ; and the same may be said of the monkeys, bats, and carnivora. Among Ungulata, however, we find, in addition to the living tapirs, llamas, peccaries, and deer, several species of horse and antelope, as well as a mastodon, all three forms due probably to recent immigration from the northern continent.
Further south, in Bolivia, the Pampas, and Patagonia, we also find abundant fossil remains, probably a little older than the cave fauna of Brazil, and usually referred to the newer part of the Pliocene period. The same