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Updated: May 14, 2025
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.
I remember, in my youth, there were detestable books which ought to have been burned by the hands of the common hangman, for they contained questions and answers to be learned by heart, of this sort, "What is a horse? The horse is termed Equus caballus; belongs to the class Mammalia; order, Pachydermata; family, Solidungula." Was any human being wiser for learning that magic formula?
In proof of the first of these positions, we may cite the fact that, in the relative development of the limbs, the civilized man departs more widely from the general type of the placental mammalia than do the lower human races.
This is all that we need ask to account for the very scanty and fragmentary group of Mammalia which now inhabit the large island of Timor.
It might have been expected, from some expressions met with here and there in the "Origin of Species," though not perhaps from a fair interpretation of the whole tenor of the author's reasoning, that this dearth of the highest class of vertebrata is inconsistent with the powers of mammalia to accommodate their habits and structures to new conditions.
These tribes are put together because comparative anatomy finds that the common characters of all cats are fundamentally like those of all dogs and bears and seals, and in these common qualities the carnivora differ from all other mammalia.
The bear takes possession of many a mountain cavern; the beaver and otter inhabit the banks of the streams and lakes; the raccoon is found in the woods; and the antelope bounds across the plains. We know more about the feathered tribes than the mammalia of Mexico. There are upwards of one hundred and fourteen species of land birds, one half of which are unknown in other parts of the world.
We have here, then, in the position of the testes in Mammalia a condition which is not in the slightest degree 'adaptive' in the ordinary sense that is, fulfilling any special function or utility. The condition must be regarded as distinctly disadvantageous, since the organs are more exposed to injury, and the abdominal wall is weakened, as we know from the risk of scrotal hernia in man.
There are certain and constant characters in their formation, which bring them all under one group, called Insectivora, or Insect-eating Mammalia, by naturalists; but among them are smaller groups of individuals, with peculiar characters, adapted to their different habits.
Hence it is that the study of this section of the mammalia is, perhaps, the most difficult of all; and a true classification of these small quadrupeds has hitherto proved a puzzle to the most expert zoologists. Of true Beavers there is only one species unless the beaver of the Old World be different from the well-known animal of the American continent.
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