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This admirable locomotorium, especially its solid central axis, the vertebral column, is a special feature of the Vertebrates, and has given the name to the group. As in other mammals, we distinguish first between the axial or dorsal skeleton and the skeleton of the limbs.
If this fails of development, as in mollusks, the group is doomed to degeneration or at best stagnation. But we have seen the dawn of a still higher function. In insects and vertebrates the brain is becoming of importance, and absorbing more and more material. This is the promise of something vastly higher and better.
The first vertebrates are sharks, ganoids and garpikes, which are the highest in structure of all known fishes. Darwin's hypothesis demands this order reversed. A painter presented a man with a red canvass, claiming that it represented the children of Israel crossing the Red sea. The question was asked, "Where are the Israelites?" The painter answered, "They have crossed over."
Now I traced some delicate nervous filament from the spinal column of the amphioxus to the cerebral hemisphere of the mammifer. Now I disclosed the ramifying canals in the vast system of circulation, mounting from the spongy network of the mollusk and the sluggish lymphatic of the reptile to the brilliant, bounding arteries of the double-hearted vertebrates.
Whether this explanation be true or not, a glance at a chart, showing the geological succession of occurrence of the different kingdoms, proves that in the oldest palæozoic periods there were well-developed cuttlefish and crabs before there were any vertebrates worthy of the name. If any were present, their skeleton was purely cartilaginous and not preserved.
In passing from the mollusk to the fish to get thence to the higher vertebrates, she turned aside in another direction toward a class of animals which rises far above mollusks, but which leads to nothing beyond.
The brain of all except the very lowest vertebrates consists of four portions: 1. The cerebrum, or cerebral lobes, or simply "forebrain," the seat of consciousness, thought, and will, and from which no nerves proceed. Whether the primitive vertebrate had any cerebrum is still uncertain. 2. The mid-brain, which sends nerves to the eyes, and in this respect reminds us of the brain of insects.
Now the lady merely smoked and chuckled. When I again uttered "Well?" with a tinge of rebuke, she came down from her musing, but into another and distant field. It was the field of natural history, of zoology, of vertebrates, mammals, furred quadrupeds or, in short, skunks. One may as well be blunt in this matter.
The supposed fact that female secondary characters in Vertebrates are absent in the male is completely disproved for Mammals by the presence of rudimentary mammary glands in the male. It is true that secondary sex-characters are usually positive in the male, while those of the female are apparently negative, but in the case of the mammary glands the opposite is the case.
We know as yet almost nothing of their history; when deciphered it will be as strange as any romance. The vertebrates are of course the most important line, as including the ancestors of man. But we must take a little glance at mollusks, including our clams, snails, and cuttle-fishes; and at the articulates, including annelids and culminating in insects.
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