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completely solved and capitalized, in his essay, Darwin." Of the sirens, commonly but erroneously reckoned among the Cetacea, and of which the majority prefer remaining at the mouths of large estuaries, one entire species has penetrated into the great inland lakes of Africa; and certain species of salmon as well as the sturgeons, which alternate periodically between salt and fresh water, are in the phase of gradually forsaking ocean life. From my special experience, I may add that the brackish-water sponges are certainly dependent on the marine families, and that the fresh-water species unmistakably point to these brackish forms.
If in all these cases we are dealing with gradual transformation, and more or less voluntary adaptation, there is no lack of conspicuous instances of forcible and almost sudden severance; of upheavals by which former sections of the ocean became inland seas. What were the modifications undergone by the fish and crabs secluded with them, is shown by the fine observations of Lovén on the animals of Lakes Wener and Wetter, and of Malmgren on those of Ladoga. The latter brings evidence that the salmon-trout of the Alps (Salmo salvelinus) is derived from the Polar Sea, and is own brother to the Scandinavian Salmo alpinus.
Rütimeyer pronounces the opinion that by more minutely tracing the relations of the fresh-water fauna to those of the denizens of the ocean, the cosmopolitanism of fresh-water animals will be explained, as well as the relation of antarctic to arctic life. For the present, however, these two great animal groups, as regards the higher, warm-blooded classes, are somewhat sharply contrasted. It is only from scanty remains that we
know that so early as the Jurassic era, the northern hemisphere was peopled by Marsupials, but, it is evident, not densely. We must suppose that, retaining their character, the Marsupials of the southern continent tested and proved their powers of adaptation, whereas on the other side of the equator a race of mammals of completely different cast proceeded from them. This is the race which still characterizes the whole surface of the earth from the north to the point of contact with the more stable remnants of antarctic life. While with reference to their origin we can appeal only to reason and inference, the historical connection between the mammalia now peopling the Old and the greater part of the New World, and their predecessors up to the most ancient Tertiary periods, is manifest to our eyes.
The remains of the earliest mammals here to be considered, are found in the Eocene deposits of Switzerland, and in corresponding strata in France and the south of England. From the southern edge of the Jurassic plateau, neither the Alps nor any other land was visible, and the ocean which washed its shores has been traced as far as China. The mammalia of this period, as far as they are known, amount, according to the synopsis made by Rütimeyer in 1867, to at least 70 species. The majority are ungulate, therefore Graminivora; of these, by far the greater number Pachydermata. Now, when the entire world scarcely maintains so many Pachyderms, this ratio is quite disproportionate. In Europe, the pig alone represents this division, and Ruminants everywhere predominate. In its present animal population, Africa might be approximately compared to Eocene Europe. But as to these Ungulates must be added a large num
ber of Carnivora, resembling the Viverrida (polecats, martens, &c.) and hyenas, and as viverridæ exist in Africa as well as in Asia, and as, moreover, the musk ruminants represented in this primitive fauna are now likewise Asiatic and African, and, finally, as the French opossums of those ages still live in Central and South America, “we gain an impression that the most ancient Tertiary fauna of Europe is the source of a truly continental animal society now represented in the tropical zone of both worlds, but most emphatically in Africa."
Far more heterogeneous is the picture of the higher animal life of the middle and more recent Tertiary periods which we reconstruct from the numerous and in parts highly prolific repositories of these remains. To draw narrower limits within these periods is impracticable; from place to place, from stratum to stratum, there is coherence; nowhere does a species appear that might not be derived from another; and our authority says that anatomy, morphology, palæontology, and geographical distribution, seemed to impress no doctrine upon him with such energy and pertinacity as that separate species of a genus, species without any historical and therefore without any previous local link to any original stock, do not exist." The most celebrated repository of Tertiary mammals is Pikermi, a short distance from Athens, an accumulation of skeletons complete and in fragments, which pre-supposes a profusion of animals, of which at any rate the most densely inhabited regions of Africa may, according to Livingstone's descriptions, give us an idea.
Again the Carnivora give way to the Graminivora,
though the feline beasts of prey make themselves conspicuous; and among the great Tertiary beasts of prey are some which have a range as great as the tiger of the present age. The territory of the extinct sabre-toothed tiger (Machairodus) at that time extended over a great part of America and Europe. Let us also mention that the canine animals appear somewhat later, and that the bears are of still more recent origin. At this period the most abundant material still favours the ungulates. Cloven feet still preponderate. Pigs and musk-animals are the most constant. But the tapir, in shape like the older forms, is now joined by the rhinoceros, the true horses, and the elephants. If the origin of the rhinoceros is somewhat obscure, the extraction of the mastodon, the older form of the elephant, is hitherto quite unknown." And yet though we search in vain through the known mammalian fauna of the Eocene period for the most nearly allied parent forms, there are numerous tokens that even in Europe and Asia, "most of the Eocene must be regarded as the true root forms of the Miocene genera." (R.) This is shown by the discoveries at Nebraska in North America, where important genera, which, like the Palæotherium, disappeared from the Old World in the Eocene period, took refuge in company with newer genera. We likewise find there, intermediate forms between the lama and the camel, which in this case alone gives its true significance to the once unmeaning word, vicarious genera. At Nebraska we moreover find the triple-hoofed horse (Anchitherium), and we hence know the origin of the single-hoofed horse of the Old and New Worlds.
What has happened in the Old World since that age
is confined to the extinction of many Pachydermata, a displacement of the rhinoceros, elephant, tapir, and hippopotamus, and an extremely abundant development of the true ruminants and the cattle which proceeded from them with an exaggerated form of head. Bears and canine species occupy the territory where viverridæ and hyenas once predominated; but as "numerous locally and historically limited species, a large number-among the smaller fauna a majority-of Miocene races remain in possession of the ancient and probably constantly increasing habitat." (R.) "In this gradual change of things, no one will be able to discern aught but phenomena of the same order of which we are still the witnesses." (R.)
How circumstances occurred in America has been described in a masterly style by Rütimeyer as follows: "America affords a basis for the distribution of animals completely different from that of the Old World. In the latter, ridges, open only in places, divide the entire continent into mountainous zones, and correspond to the distribution of temperature. Thus in a twofold manner they prescribe a definite range east and-west to the extension of animals; while a migration from north to south is impeded less by the height of the mountains than that on their summit the north comes into contact with the scorching south. Behind this wall, moreover, in the expanse from the Caspian Sea to China, there is a zone of steppes and deserts which fences in the animals more effectually than the mountain chains. In America, not beasts of prey alone, but graminivora also, may advance without hindrance from the regions of the lichen on the Mackenzie River, through the pine forests of Lake Superior, to the land of