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Although this quadrigeminum is very insignificant in man and the higher mammals, it forms a special third section, greatly developed in the lower vertebrates, the "middle brain." The narrow central canal of the spinal cord continues above into the quadrangular fourth cerebral cavity of the medulla oblongata, the floor of which is the quadrangular depression.
But as these animals and their spawn are known to be immediately killed by sea-water, on my view we can see that there would be great difficulty in their transportal across the sea, and therefore why they do not exist on any oceanic island. But why, on the theory of creation, they should not have been created there, it would be very difficult to explain. Mammals offer another and similar case.
Mastodons of several species are found in Pliocene strata in Europe and Asia; detached teeth are found in Suffolk. The Mastodons had a longer jaw and face than the elephants, though closely allied to them. So here, too, we are approaching the ordinary mammals, of which we may keep the pig and the tapir in mind as samples. But the Mastodons still had the great trunk and huge tusks of the elephants.
The atmosphere has always been continuous, and the animals that could use it as a highway had great advantages over those that could not, and so we find the slow-moving terrestrial mollusks few in number compared with the multitudinous hosts of strong-flying insects; similarly, the mammals are far outnumbered by the birds of the air, that can pass from island to island, and from country to country, unstopped by mighty rivers or wide arms of the sea.
That deficiency, as every one knows, is partly a derricked heritage from those females of the Pongo pygmaeus who were their probable fore-runners in the world; the same thing is to be observed in the females of almost all other species of mammals. But it is also partly due to the effects of use under civilization, and, above all, to what evolutionists call sexual selection.
Along with the bones were discovered the teeth of mammals, both carnivora and herbivora; also certain small perforated corals, such as were used by many ancient peoples as beads, and similar to those gathered in the deposits of Abbeville. The cave had apparently served as a place of sacrifice and of burial. In 1860 M. Lartet visited the spot.
The presence of the bones of whales and other marine mammals would prove that the strata were laid after the appearance of mammals upon earth, and imbedded relics of man would give a still closer approximation to their age. In the same way we correlate the earlier geological formations. For example, in 1902 there were collected the first fossils ever found on the antarctic continent.
Among the former, the amphibian follows the fish, the reptile follows the amphibian, the mammal follows the reptile, and non-placental mammals are followed by the placental. It almost seems as if nature hesitated whether to produce the mammal from the reptile or from the amphibian, as the mammal bears marks of both in its anatomy, and which was the parent stem is still a question.
This is only found when the essential organs of reproduction, the genital glands of both kinds, are united in one individual. As hermaphrodism was probably the original arrangement in all the Vertebrates, and the division of the sexes only followed by later differentiation of this, these curious cases offer no theoretical difficulty. But they are rarely found in man and the higher mammals.
If we compare different groups of mammals in regard to these fissures and convolutions, we find that their development proceeds step by step with the advance of mental life. Of late years great attention has been paid to this special branch of cerebral anatomy, and very striking individual differences have been detected within the limits of the human race.
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