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Updated: June 18, 2025
We seem also entitled to ask why the bats and rodents of Australia, which are spread so widely among the marsupials over that continent, have never, under the influence of the principle of progression, been developed into higher placental types, since we have now ascertained that that continent was by no means unfitted to sustain such mammalia, for these when once introduced by Man have run wild and become naturalised in many parts.
It is well to review them as they are given in the standard text-books of comparative anatomy. Primates are eutheria, or true mammalia possessing a placental attachment of the young within the parent.
On the other hand, it may be truly said in favour of progression that after all these discoveries the doctrine is not gainsaid, for the less advanced marsupials precede the more perfect placental mammalia in the order of their appearance on the earth.
The resemblance, indeed, is so striking as, on Darwinian principles, to suggest the probability of genetic affinity; and it even led Professor Huxley, in his Hunterian Lectures, in 1866, to promulgate the notion that a vast and widely-diffused marsupial fauna may have existed anteriorly to the development of the ordinary placental, non-pouched beasts, and that the carnivorous, insectivorous, and herbivorous placentals may have respectively descended from the carnivorous, insectivorous, and herbivorous marsupials.
We start from the undisputed fact that man is in any case whether we accept or reject his special blood-relationship to the apes a true mammal; in fact, a placental mammal.
There are many kinds of poison, and many varieties of those living poisons that we call microbes, which the placenta does not allow to pass through from the mother's blood-vessels into those of the child, and which are unable, fortunately for the child, to break down the placental resistance.
In the most advanced class of the stem, the mammals, there are only a few families that belong directly to our genealogical tree. Man has all these distinctive features; that is a very significant fact. We may, on the ground of the most careful comparative-anatomical and ontogenetic research, formulate the thesis: "Man is in every respect a true Placental."
If any of my readers hunger and thirst for information concerning the descent of the cat through marsupial ancestors and mesozoic mammals to the generalized placental or monodelphous carnivora of to-day, let them consult St.
Further, the most superficial study would at once convince us that, among the orders of placental mammals, neither the Whales, nor the hoofed creatures, nor the Sloths and Ant-eaters, nor the carnivorous Cats, Dogs, and Bears, still less the Rodent Rats and Rabbits, or the Insectivorous Moles and Hedgehogs, or the Bats, could claim our 'Homo', as one of themselves.
Danthez speaks of the development of a fetus in spite of the absence of an umbilical cord. Stute reports an observation of total absence of the umbilical cord, with placental insertion near the cervix of the uterus. There is mentioned a bifid funis. The Ephemerides and van der Wiel speak of a duplex funis.
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