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Updated: June 29, 2025
While our jaws and those of all other backboned animals work up and down, so that we may be said to have an upper and lower jaw, the grasshopper and all of his insect, crab, or spider relations, which have jaws at all, have them right and left, and they work from side to side. Behind these harder mouth parts is found a pair of softer jaws, each of which has on it a little finger-like feeler.
For the reasons specified, the backboned animals provide the richest arrays of these series, and such histories as those of horses and elephants have taken their places in zoölogical science as classics. But even among the invertebrates significant cases may be found.
Couch, of Polperro in Cornwall, was equally versed in the true backboned fishes; and to him Edward sent any doubtful midges, or gurnards, or gobies, or whiffs. So numerous are the animals and plants of the sea-shore, even in the north of Scotland alone, that if one were to make a complete list of all Edward's finds it would occupy an entire book almost as large as this volume.
Altogether a higher state of reproduction is encountered when we reach the reptiles, which are the next higher class of backboned animals. Here very distinct developments of the process are discovered. The turtle, to use the best known illustration, may lay but twenty eggs. But she will not lay them at random in the water, as do the toads and the fish. Each egg is wonderfully fattened with yolk.
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.
The important fact is that the second major period, called the Palæozoic, or "age of ancient animals," saw the evolution of the lowest members of the series, the invertebrates, and the most primitive of the backboned animals, like fishes and amphibia. The rocks of this long age include about 106,000 feet of strata, demanding some 21,000,000 or 22,000,000 years for their deposition.
There must have been nothing but marine plants, and these must have been on the general line of the seaweeds. Little can be definitely said concerning them. The next period of the Palæozoic is known as the Devonian age, or the age of fishes. Now the backboned animals first make their clear and unmistakable appearance.
In order that we may the more clearly understand how the mammals produce their young and nourish them, we shall begin at the lowest class of the backboned animals and note how the process is there accomplished. As we pass upward through the kingdom the method acquires greater complexity.
With the exception of the backboned animals, every important branch of the animal kingdom is represented, and it is just possible that we have even earlier forms of the vertebrates themselves. This, to the evolutionist, is very disconcerting.
I hailed him as the only other backboned friend who shared the root-world with me, and then listened to a high, sweet tone, which came forth in swinging rhythm. It took some time for my eyes to become accustomed to the semi-darkness, and then I saw what the gecko saw a big yellow-bodied fly humming in this cavern, and swinging in a small orbit as she sang.
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