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Updated: May 18, 2025
It becomes a most important matter to ascertain the limits of the placental power, and by observation upon human beings and experiment upon the lower animals this matter has been very thoroughly elucidated of late years.
Science with its hammer and chisel may lay bare evidence, may collate material, date man's appearance, call him the most recent of placental mammals, trace his superstitions and his first conceptions of a god from the elemental fears of the savage. But the mystic turns aside with an assumption of superior knowledge; he waves away objective evidence; he has a certainty impressed upon his mind.
Further search in the unexplored geological treasures and dense forests of Africa is needed. We may provisionally conceive the placental mammals as a group of the South African early mammals which developed a fortunate variation in womb-structure during the severe conditions of the early Mesozoic.
That we do not know all about the interchange of substances between mother and child must be admitted; but the essential facts, and they alone are of interest here, have been established beyond contention. There is no doubt whatever that the mother's blood surrounds the placental villi but never enters the child.
After these are born, the rest of the body slips easily into the world, and the second stage ends. THE PLACENTAL STAGE. Although the third stage is chiefly concerned with the separation and the delivery of the after-birth, on which account it is known as the placental period, the description of other no less remarkable events belongs here.
And yet the persecution, the impossibility of contending by brute strength, may have forced the mammal into the line of brain-building and placental development. The early development of mammals appears to have been slow. Palæontology proves that they were long surpassed by reptiles and birds. But the little mammal had the future. The battle was to go against the strong. Once again.
In the Oligocene we find primitive squirrels, beavers, rabbits, and mice. The Insectivores also developed some of the present types at an early date, and have since proved so unprogressive that some regard them as the stock from which all the placental mammals have arisen. Some specimens of it are seventy feet in length.
This fact made him doubt whether the Amphitherium might not be an insectivorous placental, although it offered some points of approximation in its osteology to the marsupials, especially to the Myrmecobius, a small insectivorous quadruped of Australia, which has nine molars on each side of the lower jaw, besides a canine and three incisors. Phascolotherium Bucklandi, Broderip, sp. a.
Seeing, as we do, that in large areas of the Pacific this is a period characterized by abundance of corals; that in the North Atlantic it is a period in which a great chalk-deposit is being formed; and that in the valley of the Mississippi it is a period of new coal-basins seeing also, as we do, that in one extensive continent this is peculiarly an era of implacental mammals, and that in another extensive continent it is peculiarly an era of placental mammals; we have good reason to hesitate before accepting these sweeping generalizations which are based on a cursory examination of strata occupying but a tenth part of the Earth's surface.
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