Mafi in the Light of Evoltttion 



Crustacea were all experimenting with the skele- 

 ton. The vertebrate developed an internal loco- 

 motive skeleton of little use for protection. The 

 mollusk had gained an external, purely protec- 

 tive skeleton or shell. The crustacean followed 

 the safer middle course. Its skeleton was mostly 

 for locomotion, but somewhat for protection. 



The external skeleton was easy to build. The 

 cells of the skin of many marine forms secrete 

 carbonate of lime or horn. The covering was 

 likely to vary because external. Every form 

 having a little larger and more effective pro- 

 tective skeleton gained thereby a very tangible 

 and immediate advantage and was fostered by 

 natural selection. The evolution of such an or- 

 gan goes on very rapidly. The main groups of 

 mollusks, with their most Important characters, 

 had all appeared in early paleozoic time. 



An internal skeleton, an organ of locomotion, 

 was far more difficult to build. New tissues, car- 

 tilage or gristle, and finally bone, had to be 

 formed. The deep-seated organ, being less ex- 

 posed to varying conditions, varied less. Very 

 slight variations would probably give less advan- 

 tage in the struggle, and hence were more slowly 

 accumulated through natural selection. For all 



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