The Survival of the Fittest 



these reasons the molluscan skeleton was prac- 

 tically complete, conferring all its great advan- 

 tages, before the vertebrate skeleton was more 

 than begun. The mollusk had attained very tan- 

 gible results while the vertebrate was scarcely 

 more than a bundle of possibilities. 



The protective shell of the mollusk hindered 

 locomotion, and thus hampered the development 

 of muscle and nerve. It kept the body short 

 and unfit for rapid movement. But muscle and 

 nerve were the two tissues capable of great com- 

 plexity and high development. They develop 

 through their use in locomotion. The external 

 shell, having few possibilities in itself, and de- 

 stroying those of other organs, was fatal to any 

 high development of the molluscan type. The 

 evolution of mollusks went on very rapidly at 

 first, and then ceased almost completely. There 

 was abundant variation within somewhat narrow 

 limits, there was easy and sure survival, but there 

 was little progress anywhere. The clam is the 

 logical goal of such an experiment. 



The vertebrate skeleton, comparatively sim- 

 ple at first, was constantly improved and addi- 

 tions were made to it. But, only when it had 

 attained a certain size and strength, could it 



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