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Updated: May 24, 2025
If the three localities in Europe where the most ancient mammalia have been found Purbeck, Stonesfield, and Stuttgart had belonged all of them to formations of the same age, we might well have imagined so limited an area to have been peopled exclusively with pouched quadrupeds, just as Australia now is, while other parts of the globe were inhabited by placentals; for Australia now supports one hundred and sixty species of marsupials, while the rest of the continents and islands are tenanted by about seventeen hundred species of mammalia, of which only forty-six are marsupial, namely, the opossums of North and South America.
It has long been one of my fancies that this country is an epitome of the natural world, and that if any one has come really into contact with its productions, and is familiar with them, and what they mean and represent, then he has a knowledge of all that exists on the earth. It holds good even of Australia; for palaeontologists produce fossil remains of marsupials or kangaroos.
Therefore, we must suppose either that all Rodents, including the bizcacha, branched off from some ancient Marsupial, which will naturally have been more or less intermediate in character with respect to all existing Marsupials; or that both Rodents and Marsupials branched off from a common progenitor, and that both groups have since undergone much modification in divergent directions.
Thus there is a vast negative difference between the Cainozoic and the Mesozoic mammalian faunae of Europe. But there is a still more important positive difference, inasmuch as all these Mammalia appear to be Marsupials belonging to Australian groups, and thus appertaining to a different distributional province from the Eocene and Miocene marsupials, which are Austro-Columbian.
They are gentle little creatures, with huge, round eyes, long, delicate ears, and tails tufted at the end like the feathers on an arrow's shaft. The name expresses exactly what they are like diminutive kangaroos but, of course, they are rodents and not marsupials.
In this respect there is a very conspicuous difference between the lower and higher mammals. In the Marsupials, in which the embryo is only a comparatively short time in the womb and is born in a very immature condition, the vascular arrangements in the yelk-sac and the allantois suffice for its nutrition, as we find them in the Monotremes, birds, and reptiles.
He tries to explain the phenomenon on the basis of the multiple origin of the mammals; and in fact there is already speculation regarding triple origin, viz: tambreets, marsupials, and the other mammals. Now if the latter also possessed a multiple origin, the problem of the extinction of the saurians would, according to Steinmann solve itself.
Waterhouse, of all Rodents, the bizcacha is most nearly related to Marsupials; but in the points in which it approaches this order, its relations are general, and not to any one marsupial species more than to another. As the points of affinity of the bizcacha to Marsupials are believed to be real and not merely adaptive, they are due on my theory to inheritance in common.
On the other hand, of all Marsupials, as Mr. Waterhouse has remarked, the phascolomys resembles most nearly, not any one species, but the general order of Rodents. In this case, however, it may be strongly suspected that the resemblance is only analogical, owing to the phascolomys having become adapted to habits like those of a Rodent.
From these various considerations it is probable that the Simiadae were originally developed from the progenitors of the existing Lemuridae; and these in their turn from forms standing very low in the mammalian series. The Marsupials stand in many important characters below the placental mammals.
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