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Of course we shall never know what narrow escapes our race had from extinction in the remote past; some forms have ended in a blind alley, like the sea-urchin and the oyster. Arthropoda have continued to evolve and have reached their high-water mark of intelligence in bees and ants. The vertebrates went forward and have culminated in man.
The line of development which the insect followed was comparatively easy and its course probably rapid. Certain crustacea, aquatic arthropoda, are among the oldest fossils, and it is possible that insects lived on the land before the first fish swam in the sea.
But A soon broke up into two principal stirpes, of which one, a, became the root of the Annelida, Echinodermata, and Arthropoda, while the other, b, gave rise to the Polyzoa and Ascidioida, and produced the two remaining stirpes of the Vertebrata and the Mollusca.
When, however, we come to the Arthropoda and Vertebrata, even to the lowest classes in these two great Sub-Kingdoms, sexual selection has effected much. In the several great classes of the animal kingdom, in mammals, birds, reptiles, fishes, insects, and even crustaceans, the differences between the sexes follow nearly the same rules.
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