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Updated: May 24, 2025
These creatures are classed by some naturalists among the Monkeys by reason of being "four-handed," while others are disposed to consider as still more important their marked relationship with, and affinity to, the marsupials, gnawers and insect-feeders. On the whole, these creatures are strangely organized and come very near to being a "connecting-link" between other forms.
The three lowest orders of mammals, namely, marsupials, edentata, and rodents, co-exist in South America in the same region with numerous monkeys, and probably interfere little with each other.
Waterhouse, of all rodents the vizcacha is most nearly related to marsupials; but in the points in which it approaches this order its relations are general, that is, not to any one marsupial species more than to another. As these points of affinity are believed to be real and not merely adaptive, they must be due in accordance with our view to inheritance from a common progenitor.
This pressure has finally culminated in forcing the sacrum of the European nearer the pubes, with consequent lateral expansion and contraction of the diameter from front to back. From the marsupials to the lemurs the box-shaped pelvis remains.
Thus, in South America and in Australia, the later tertiary or quaternary fossils show that the fauna which immediately preceded that of the present day was, in the one case, as much characterised by edentates and, in the other, by marsupials as it is now, although the species of the older are largely different from those of the newer fauna.
If this be correct, there can be little doubt that they were gained or at least modified through sexual selection; but how far the same view may be extended to other mammals is doubtful. I will first give briefly all the cases known to me of male quadrupeds differing in colour from the females. With Marsupials, as I am informed by Mr.
They appeared at an earlier geological period, and their range was formerly much more extensive than at present. Hence the Placentata are generally supposed to have been derived from the Implacentata or Marsupials; not, however, from forms closely resembling the existing Marsupials, but from their early progenitors.
Among the twelve or fourteen species of Mammalia which are said to have been found in the Purbecks, not one is a member of the orders Cheiroptera, Rodentia, Ungulata, or Carnivora, which are so well represented in the Tertiaries. No Insectivora are certainly known, nor any opossum-like Marsupials.
There was a time when huge bird-like reptiles were the lords of creation, and after these had been "sealed within the iron hills" there came successive dynasties of mammals; and as the iguanodon gave place to the great Eocene marsupials, as the mastodon and the sabre-toothed lion have long since vanished from the scene, so may not Man by and by disappear to make way for some higher creature, and so on forever?
But in the Marsupials a communication is opened between the two Mullerian ducts, and in the Placentals they combine below with the rudimentary Wolffian ducts to form a single "genital cord." The original independence of the two wombs and the vaginal canals formed from their lower ends are retained in many of the lower Placentals, but in the higher they gradually blend and form a single organ.
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