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The familiar idea of world-enveloping strata as representing successive ages was not discarded; but instead of Werner's successive ages of limestone making, sandstone making, etc., these new investigators taught that there were successive ages of invertebrates, fishes, reptiles, and mammals, these creatures having registered their existence in rocky strata which thus by hypothesis completely encircled the globe one outside another.
Comparative anatomy has asserted that this is so, as we have already learned, for it finds that adult animals array themselves at different levels of a scale beginning at the bottom with the protozoa, continuing on to the two-layered animals like Hydra and jellyfish and sea-anemones, and then extending upwards to the region of the more complicated invertebrates and vertebrates.
We will try to reproduce briefly the pedigree which is of most interest the hypothetical pedigree of man. Häckel divides it into twenty-two stages, eight of them belonging to the series of the invertebrates, and fourteen to that of the vertebrates.
"I will meet you at San Miguel's to-morrow afternoon, Gail," Eloise said, as we reached a low but pretentious adobe dwelling. "This is my home now." "Your new Mexican homes are thick-walled, and you live all on the inside," I said, as we paused at the doorway. "They make me think of the lower invertebrates, hard-shelled, soft-bodied animals.
We proceed to investigate the points at which biology touches our problem, and again three barriers call for notice and investigation: The difference between plants and animals; the difference between vertebrates and invertebrates; and the difference between mammals and all other vertebrates. Whence the animal kingdom?
Just what evidence is there for this conception? In the first place, there is the fascinating story of the origin of vertebrates from invertebrates of the sea scorpion or spider type.
"There is light," he said at last, "in spite of all invertebrates. Good-bye! I've wasted enough time," and he abruptly strode away. "And in spite of myopia?" muttered Hilary. A few minutes later, coming out from Messrs. Rose and Thorn's, where he had gone to buy tobacco, he came suddenly on the little model, evidently waiting.
By means of this strength, and ease, these forms are enabled to move rapidly in pursuit of their prey, and away from their pursuers, and also to resist outside pressure or attack. They are protected in a way similar to the invertebrates having shells, and yet have the additional advantage of easy movement.
Until 1866 no one had any idea of the close connection of these apparently very different animals; it was a very fortunate accident that the embryology of these related forms was discovered just at the time when the question of the descent of the vertebrates from the invertebrates came to the front.
These cuttles are the only invertebrates at all in the running so far as colossal size is concerned, and it will be observed that here the largest modern specimen immeasurably beats the largest fossil form of the same type.
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