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The central nervous system, with the predominance given to the ganglia at the fore end of the nerve cord, the brain, already endowed them with greater energy than the invertebrates; and, still more important, these structures contained the possibility of development into the more highly organized land vertebrates which were to rule the earth.
I do not say that there were not fossil forms quite as big as the gigantic calamaries of our own time on the contrary, I believe there were; but if we go by the record alone we must confess that, in the matter of invertebrates at least, the balance of size is all in favour of our own period.
That the five main-types of the invertebrates seem to have appeared quite contemporaneously, yet that the zoöphytes really appeared first, does not contradict the before-mentioned law of a progress in the appearance of the organisms from the lower to the higher.
He has struggled upward through the invertebrates, through the fish, through the reptile, through the lower mammals, through his simian ancestors till he reaches his goal in the man we know. Darwin was not the author of the theory of evolution, but he made the theory alive and real to the imagination. He showed us what a master key it is for unlocking the riddle of the life of the globe.
But the earlier record is, for reasons I have already given, obscure; and as my purpose is rather to discover the agencies of evolution than to strain slender evidence in drawing up pedigrees, I need only make brief reference to the state of the problem. Until comparatively recent times the animal world fell into two clearly distinct halves, the Vertebrates and the Invertebrates.
In the lowest vertebrates and in the homologous tissues of the higher invertebrates, the fractions of the thyroid are intimately connected with the ducts of the sexual organs. They are indeed accessory sexual organs, uterine glands, satellites of the sex process.
Most invertebrates and many of the lowest two classes of backboned animals, the fishes and the amphibians, cannot merely stop up a rent, but renew an entire limb, fin, yes, even eye or head. Cut an earthworm in two and the rear half will grow a new head and the front half a new tail.
Gradually the antique types of the Paleozoic fauna died out, and in the Permian rocks are found the last survivors of the cystoid, the trilobite, and the eurypterid, and of many long-lived families of brachiopods, mollusks, and other invertebrates. The venerable Orthoceras and the Goniatite linger on through the epoch and into the first period of the succeeding era.
That line which is most nearly upright will represent the evolution of the highest form of vertebrate, except man; the next, the next highest; and so on till you come to the lines representing the invertebrates; and so on till you come to the lines which are getting nearer and nearer to the horizontal.
Of course, our knowledge of the life of the invertebrates, save the termites, the ants, and the bees, is extremely limited; and yet, even as regards the lower animals, we may glean a few facts of well-ascertained cooperation.
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