m. 644] 



ARRESTED EVOLUTION 



257 



Dipnoi, Teleostei, Reptilia eacli have one in the 1896 

 edition of Zittel-Eastman. The vertebrate volume had 

 not yet appeared of the second edition. 



From an analysis of the percentages we drew the fol- 

 lowing inferences : 



1. The lower classes tend in general to have more per- 

 sistent types than the higher. 



2. Within each order and class, again, the lower sub- 

 classes tend to furnish the greater percentage of per- 

 sistent forms. 



3. Frequently the persistent genera form a primitive 

 central stock from which numerous shorter lived genera 

 branch off. 



4. The stable conditions of the open ocean and deep 

 sea (as in the Foraminifera) and the subterranean con- 

 ditions favor persistence of types, the latter condition 

 including the burying and boring forms. 



5. Sessile forms contain more persistent types than 

 the vagile benthos. 



6. Persistent tj^es prevail in much greater number 

 among the marine forms than among the land and fresh- 

 water animals. Among the continental forms again the 

 limnal and fluviatile forms appear to be more persistent 

 than the terrestrial forms. 



7. Most persistent tji^es are small and inconspicuous 

 forms. 



8. Many persistent genera show a slow development, 

 a distinct climacteric period and a long post-climacteric 

 period. Connected with this observation is the other 

 that persistent genera which slowly develop never pro- 

 duce many species during a single geologic period. 



9. Minor factors of persistence are seen in (a) extreme 

 individual vitality (as in Lingula and Crania), (b) im- 

 mense broods (as in Ostrea and Limuliis), (c) extreme 

 restriction in the matter of food, as in the eaters of 

 carrion and refuse (CapulidjB, oyster, etc.). 



The same criteria were found to hold, on the whole, 

 in regard to the persistent species and the higher groups 



