May 4, 1921 clark: evolution of animals 207 



book and mind enriched with all that he saw, so that the by-products 

 may eventually outvalue the answer to his original problem. Mr. 

 Gilbert well illustrated the geologist whose researches yielded so great 

 a wealth of products, and in his work the line between pure geology 

 and useful geology was not marked. In that type of geology, broad 

 in scope and accurate in detail, by-products of high value can always 

 be expected. 



ZOOLOGY. — The steps in the evolution oj animals} Austin H. 

 Clark, U. S. National Museum. 



In the following dichotomous table are included all the essential 

 characters of all the major animal groups, showing the successive 

 steps by which the two most specialized phyla, the Vertebrata (in 

 heavy-faced type) and the Arthropoda (in italics), have been evolved, 

 and at the same time the relationship to the main developmental 

 line of all the numerous off-shoots which have proved incapable of 

 further evolution. 



This table, which is based upon the body structure of adults, 

 assumes the evolutionary course to have run as follows: 



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 correlated development 

 of a head end overcame the inefficiency 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 stage in the 

 centralization and perfection of the previously unorganized respiratory 

 system. 



6. The development of the dorsal nerve cords led to increased ner- 

 vous efficiency through the unification and concentration of the 

 nervous system. 



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

 w^as the final step in the centralization of the nervous control of the 

 body. 



1 Received March 9, 1921. 



