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Updated: June 24, 2025
This fact would appear to be quite inconsistent with the theory above proposed, but it must be remembered that the ovum of Monotremes is known to remain for a short period in the oviduct, or in other words to pass through it very slowly, and to absorb fluid from its walls, as shown by the considerable increase in size which the ovarian ovum undergoes before it is laid.
Think, my friends, of a continent, the margin of which, instead of the center, rose out of the waves originally like a gigantic ring, which encloses, perhaps, in its center, a sea partly evaporated, the waves of which are drying up daily; where humidity does not exist either in the air or in the soil; where the trees lose their bark every year, instead of their leaves; where the leaves present their sides to the sun and not their face, and consequently give no shade; where the wood is often incombustible, where good-sized stones are dissolved by the rain; where the forests are low and the grasses gigantic; where the animals are strange; where quadrupeds have beaks, like the echidna, or ornithorhynchus, and naturalists have been obliged to create a special order for them, called monotremes; where the kangaroos leap on unequal legs, and sheep have pigs' heads; where foxes fly about from tree to tree; where the swans are black; where rats make nests; where the bower-bird opens her reception-rooms to receive visits from her feathered friends; where the birds astonish the imagination by the variety of their notes and their aptness; where one bird serves for a clock, and another makes a sound like a postilion cracking of a whip, and a third imitates a knife-grinder, and a fourth the motion of a pendulum; where one laughs when the sun rises, and another cries when the sun sets!
This and other causes acted to place the Placentals in the "Royal line" from which Man was evolved. The following families of Placental Mammals are recognized by Science, each having its own structural peculiarities: The Edentata, or Toothless creatures, among which are the sloths, ant-eaters, armadillos, etc. These animals seem to be closer to the Monotremes than they are to the Marsupials;
But the three sub-classes differ so widely from each other, not only in the construction of the sexual organs, but in many other respects also, that we may confidently draw up the following important phylogenetic thesis: The Monodelphia or Placentals descend from the Didelphia or Marsupials; and the latter, in turn, are descended from the Monotremes or Ornithodelphia.
When we pass on to the fishes, we find their intelligence remaining at a very low level. We do not see any material advance in mental development until we go on to the Amphibia and Reptiles. There is still greater advance when we come to the Mammals, though even here the minds of the Monotremes and of the stupid Marsupials remain at a low stage.
It is then clear that the Marsupials viviparous Mammals without placenta are a necessary transition from the oviparous Monotremes to the higher Placentals with chorion-villi. In this sense the Marsupial class certainly contains some of man's ancestors.
In them the milk comes out through a flat portion of the ventral skin that is pierced like a sieve, as we still find in the lowest living mammals, the oviparous Monotremes of Australia. The young animal licks the milk from the mother instead of sucking it. In many of the lower mammals we find a number of milk-glands at different parts of the ventral surface.
The thin wall of the allantois consists of the same two layers or membranes as the wall of the gut the gut-gland layer within and the gut-fibre layer without. This is the case also with the lowest mammals, the oviparous Monotremes and most of the Marsupials. The latter also is richly permeated with blood-vessels which bring the mother's blood to the embryo.
It seems clear, at least, that two main branches, the Monotremes and Marsupials, arose from the primitive mammalian root. Whether either of these became in turn the parent of the higher mammals we will inquire later.
Nor have they the marsupial bones in the ventral wall at the anterior border of the pelvis, which the Marsupials have in common with the Monotremes, and which are formed by a partial ossification of the sinews of the inner oblique abdominal muscle. There are merely a few insignificant remnants of them in some of the Carnivora.
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