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creation of the perfected animal type, animal development 

 began so to speak, anew, and was rapidly advanced ; the 

 development of a skeleton enabled the muscles to function 

 to better advantage, leading to a more perfect coordination 

 of muscular action and to greater intensity of bodily activity; 

 the appearance of gill clefts indicated the first stage in the 

 centralization and perfection of the previously unorganized 

 respiratory system ; the development of the dorsal nerve cords 

 led to increased nervous efficienc}^ through the unification and 

 concentration of the nervous system ; and the development 

 in connection with the last of a specialized head was the final 

 step in the centralization of the nervous control of the body. 



In the following key are included all the major animal groups 

 arranged according to the progressive economic efficiency 

 of their bodily structure, the several steps in advance being 

 assumed to be the following. 



1. The formation of a multicellular body overcame the 

 limitation in size inseparable from the unicellular body. 



2. The appearance of quadrilateral symmetry (with a right 

 and left side and a dorsal and ventral surface) and the corre- 

 lated development of a head end overcame the ineffiiciency 

 of a radially symmetrical body. 



3. The development of a vascular and of a respiratory 

 system facilitated the repair of waste and led to a marked 

 increase in bodily activity. 



4. The development of a skeleton enabled the muscles 

 to function to better advantage, leading to a more perfect 

 coordination of muscular action and greater perfection of bodily 

 activity. 



5. The appearance of gill clefts indicated the final^ 

 in the centralization and perfection of the previousl 

 ganized respiratory system. 



6. The development of the dorsal nerve cords 

 increased nervous efficiency through the unification and 

 tration of the nervous system. 



7. The development in connection with the last, of a definite 

 head was the final step in the centralization of the nervous 

 control of the body. 



(400) 



