Man in the Light of Evoltttion 



a protective skeleton also. Some mollusks re- 

 duced or cast off the shell. Each form of skele- 

 ton was an experiment by Itself. Each was tried 

 with a great variety of modifications. The ex- 

 periments were extensive and exhaustive. Every 

 line was made or marred by the skeleton which 

 it developed. 



The only completely successful experiment 

 proved to be that of vertebrates. Every other 

 form of skeleton had some fatal defect. The 

 extreme logical result of molluscan development 

 is shown by the clam. He developed a bivalve 

 shell and burrowed in the mud. He thus gained 

 almost complete protection from his enemies, 

 abundant food, rapid increase in numbers, and 

 nearly unchanging surroundings. Having once 

 adapted himself to the mud, further develop- 

 ment was entirely useless. But the shell ham- 

 pered or prevented locomotion, made most of 

 the senses unnecessary, stopped the development 

 of muscle and nerve, and made further progress 

 Impossible. Amid all the changes of the vast 

 geological periods the clam has slumbered peace- 

 fully in inexpressible comfort. 



The external locomotive skeleton of the insect 

 favors muscular and nervous development. In- 



30 



