THE EVOLUTION OF ORGANIC FORM. 93 



As we ascend to the higher members of the Articulate type, the 

 specialization of function increases, but not sufficiently to obliterate all 

 the individuality of the segments. The tendency in the Arthropods is 

 toward a continuous welding of the segments. Thus, in the Crusta- 

 ceans we find twenty or twenty-one segments compacted into three 

 body sections. In the higher families these three are reduced to two ; 

 and, in the highest crabs, the abdominal section becomes so reduced 

 that all the body functions are performed by a single section. 



At the same time the chitinous armor of the segments becomes a 

 continuous cortical armor ; and the chain of nerve-ganglia is reduced 

 to a single large ganglion, which supplies nerve-fibers to all the body. 



In this manner the Crustacean reaches its most specialized condi- 

 tion, but only by a loss of its longitudinal extension, and a return to 

 the compact, slow-moving, armored type of animal. Thus, its highest 

 evolution has produced an organization antagonistic to any advanced 

 degree of development. 



Of the air-breathing Arthropods the Arachnidoe seem to be closely 

 allied to the Crustaceans. They have the same compactness of organi- 

 zation ; are not, as a rule, adapted to swift motion ; and are inferior 

 to the high Crustaceans from the fact that their tendency is toward 

 development of the abdomen, instead of the head section, as in the 

 crabs. 



The insects and Myriapods do not possess the relations to the Crus- 

 taceans shown by the spiders and their allies. On the contrary, their 

 larval form seems to indicate a separate line of descent from the 

 worms. Different as insects and Myriapods are in their mature forms, 

 they appear to have had a common origin the embryo of the Myria- 

 pods passing through a stage that resembles the larval stage of insects. 

 They seem, indeed, to have developed from their primitive form in 

 opposite directions, the segments being multiplied in the Myriapods 

 and reduced in the insects. The embryo of the Myriapod has at first 

 but three pairs of legs. At a later period posterior legs bud succes- 

 sively from the new-formed segments. There seems to be no fixed 

 limit to the number of segments, since they continue to increase 

 throughout life. And their individuality is strongly declared, each 

 segment possessing the organs necessary to a separate life, as a nerve- 

 ganglion and fibers, breathing organs, muscles, an intestine, and a vas- 

 cular space. These organs, if redeveloped from their partly aborted 

 condition, might well suffice to sustain life in separate animals. 



Even in the highest of the Arthropods the insects this heredi- 

 tary individual organization of the segments continues manifest ; these 

 organically independent members of the society stubbornly resist the 

 cession of their primitive functions, only partly yielding to the com- 

 mon needs, and thus retaining a generalization of function which is 

 repressive of any high development. 



The animal best suited for progression is one which has all its 



