82 PRIMARY FACTORS OF ORGANIC EVOLUTION. 



This progress has not been accomplished without 

 much degeneracy by the way. All of the branches 

 display divisions which have become sessile, and some 

 of them are almost altogether so. Among Ccelenterata 

 the Actinozoa are fixed, and often develop a calcareous 

 skeleton. Many of the Hydroids are sessile. The 

 great branch of the Echinodermata has its locomotive 

 powers greatly curtailed, and many of them are per- 

 manently sessile. The same is true of the Mollusca. 

 Both divisions are at one side of the line of progres- 

 sive evolution as a consequence of this tendency. The 

 Vermes display in their free representatives the condi- 

 tions of progressive evolution. Being longitudinal 

 and bilateral, one extremity becomes differentiated by 

 first contact with the environment, as the seat of spe- 

 cial senses, the basis having been secured by the loca- 

 tion there of the nervous centers and ring. The Ar- 

 thropoda present us with a great development of 

 locomotive organs, and of special senses. As a whole, 

 they have not made a considerable advance into the 

 possession of the higher animal mental capacities, but 

 display various degeneracies or degenerate tendencies 

 among themselves. The moderately specialized as to 

 structure are the most intelligent. These are the Hy- 

 menoptera, which display mental capacity superior to 

 that of many Vertebrata. The latter branch, although 

 presenting one sessile type, the Urochorda, has pro- 

 duced in its highest class, the Mammalia, the most gen- 

 eral elevation in this, the highest of animal functions. 

 This intelligence is in most of the types expended in 

 preserving themselves from destruction against hostile 

 environments, and the conquest of nature thus effected 

 is remarkable from a physical point of view, but is an 

 end of no great elevation of purpose from a mental 



