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As for the backboned animals which are cold-blooded, these must, unless they are fish, give up the struggle completely, bury themselves in out-of-the-way places, and go worse than dormant. They often become absolutely cold and stiff.
This is a member of that curious family of Ascidians, which forever trembles in the balance between the higher backboned animals and the lower division, where are classified the humbler insects, crabs, and snails.
There were no backboned animals, that is to say, no land mammals, reptiles, or fishes at this stage of the earth's history. It is not likely that there was any land life except of plants and those forms like the lowest ferns, and probably mosses.
So far the examples have been taken from one order of the highest class of backboned animals, called mammalia. When our survey is extended to other divisions of this class, additional laws of organic relationship are discovered.
The creative energy manipulates a fish and it turns into a reptile; it covers a mollusk as with a vapor and behold, a backboned creature instead! Now we see a little creature no larger than a fox and when we look again, behold the horse; a wolf or some kindred animal is plunged into the water, and behold, the seal!
It is evident that, in what is called the evolution of animal forms, the foot came in suddenly when the backboned creatures began to live on the dry land that is, with the frogs. How it came in is a question which still puzzles the phylogenists, who cannot find a sure pedigree for the frog.
But all of these diverse forms are true fishes, possessing a common heritage of structure which demonstrates their unity of origin. The brief review of backboned animals has shown how comprehensive are the principles of relationship.
"That would be awfully interesting!" said Colin. "It will teach you a lot," rejoined the Deputy Commissioner, "and you can't ever know too much about sea-life. The real backboned fishes, with which the Bureau principally deals, are only a small part of the population of the ocean." "Shall I go and call on this gentleman, then, Dr. Crafts?" the boy asked.
Even the poles carried by canal boats to bear windmills must be regarded as the reduced vestiges of masts originally constructed to carry sails; and their adaptive evolution, like that of countless structures in animals, has been accomplished by degeneration. The birds are another class of backboned animals which exhibit identical principles of relationship.
When we come to the backboned animals there is a little more tendency to a stationary location. The sun fish may frequent the same reach of the stream, the trout may haunt the same pool, year after year, but a great majority of fishes doubtless move indiscriminately up and down the stream or about the lake or ocean and are not found two successive days in the same place.
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