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But this must not be understood in the sense that the existing Amphioxus, or the sharks or amphibia of to-day, can give us any idea of the external appearance of these remote stem-forms. Still less must it be thought that the Amphioxus or any actual shark, or any living species of amphibia, is a real ancestral form of the higher vertebrates and man.
To the doctor's immense satisfaction, the brain whose loan he was enjoying responded to the question. "On Saloni, the vertebrates have not yet appeared. None but the lowest forms of life have been found." "Is this planet larger or smaller than ours, Mr. Ernol?" "Larger. It will be a matter of millions of centuries before such beings as humans are evolved there." "How do we know these facts?"
S. V. Clevenger considers these organs to have had a branchial or respiratory origin, saying that there are many reasons for believing them to be rudimentary gills. Owen says that the thymus appears in vertebrates with the establishment of the lung as the main or exclusive respiratory organ. It is wanting in all fishes, also in the gill-bearing batrachians, siren and proteus.
In the highest division, the Vertebrates, so characteristically called by Baer the Doubly Symmetrical type, a solid column runs through the body with an arch above and an arch below, thus forming a double internal cavity.
Although he saw that animals of successive periods displayed similar structures, like the skeleton of vertebrates, which testified to some connection, Cuvier could not bring himself to believe that this connection was a genealogical one.
They show us that all the higher Vertebrates reproduce in their earlier stages, in harmony with the biogenetic law, the process that had so important a part in the rise of the whole Chordonia-stem.
Here we have the first appearance of the division of the alimentary tube into two sections that characterises the Chordonia. The differentiation of these two parts of the gut in the Enteropneust is just the same as in all the Tunicates and Vertebrates.
A. Farre's well-known article in the 'Cyclopaedia of Anatomy and Physiology, vol. v. 1859, p. 642. Owen, 'Anatomy of Vertebrates, vol. iii. 1868, p. 687. No one will pretend that so perfect a structure as the abnormal double uterus in woman could be the result of mere chance.
We should naturally expect to find the Vertebrates introduced in their simplest form; but this is by no means the case: the common fishes, as Cod, Herring, Mackerel, and the like, were unknown in those days. But there are two groups of so-called fishes, differing from these by some marked features, among which we may find the modern representatives of these earliest Vertebrates.
It is well known that we find in many insects a degree of intelligence that is found in man alone among the Vertebrates. Everybody knows of the famous communities and states of bees and ants, and of the very remarkable social arrangements in them, such as we find among the more advanced races of men, but among no other group of animals.
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