brates. Whatever size it might have attained in the absence of the Vertebrata, it certainly would be unfitted to compete with these better adapted animals for the possession of the higher fields of life.
Insects thus seem restricted to a small form, contracted localities, and a narrow range of conditions. The ants, their highest form, is one of the most limited in range. It is highest in having best succeeded in adapting nature to its needs, and, in so doing, having developed a superior mentality; but it can not advance beyond the needs of its contracted environment.
In the various animal types we have considered, Nature seems to have exhausted all side-issues in her efforts to produce an animal form adapted to a high grade of evolution. The persistent individuality of the segments hinders a colony from merging into an individual capable of an advanced phase of development.
Another and simpler method remains to be considered; the direct elongation of a single individual—not the elongation of a previously organized animal, but a primary derivative, unshackled by anatomical difficulties.
For high progress in this individual, certain conditions are necessary. It must not seek safety in a coat of armor. It must save itself from danger by powers of flight and acuteness of sense. In a water residence the most effective flight is gained by swimming. Therefore our worm must become a swimming animal, its sides being flattened into swimming-flaps.
In such an individual the functions would be specialized, as they were in the individuals which became welded into the Articulate. Indeed, the Vertebrate and the Articulate may have had a single origin in this primitive organic form.
The swimming worm we are considering has no hindrances to specialization of function. His side-flaps may be reduced to local fins. His intestinal tube—not acting as a series of sectional stomachs—may become localized in function, its anterior portion acting as a lung, its posterior portion as a stomach. There are several advantages in this. The circulation is no longer exposed to danger by a perilous thinning of the outer surface into branchiæ. The food being drawn in by water-currents, oxygen is extracted from the water by the anterior intestine, and aliment by the posterior. Similarly, the nerve and muscle systems are single and specialized, and the sense organs local.
But another condition is necessary to the full adaptation of this swimming animal to its situation. Its swift motion necessitates muscular vigor, and requires some firm point of attachment for the muscles. In all the armored types the shell, or outer coating, serves for this purpose. In the naked worm there is no such exterior point of attachment, and an interior one must be developed.
Thus we have arrived at the necessity of an interior skeleton, an
