THE EVOLUTION OF ORGANIC FORM. 95 



brates. Whatever size it might have attained in the absence of the 

 Vertebrata, it certainly would be unfitted to compete with these bet- 

 ter 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 suc- 

 ceeded in adapting nature to its needs, and, in so doing, having devel- 

 oped 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 capa- 

 ble 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 neces- 

 sary. 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 ori- 

 gin in this primitive organic form. 



The swimming worm we are considering has no hindrances to spe- 

 cialization 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 branchiae. 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 mus- 

 cular vigor, and requires some firm point of attachment for the mus- 

 cles. 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 



