REPORT OF THE DIRECTOR I916 II7 



great mass of the organic types, would still remain true and invite 

 investigation of its underlying causes. 



General Inferences from Tables 



From a general survey of the percentages of persistent types of 

 the classes and of their subdivisions, we may draw the following 

 inferences : 



1 The lower classes tend in general to have more persistent 

 types than the higher. This is exemplified by the higher percen- 

 tages of the Foraminifera, Corals, MoUuscoidea and MoUusca in 

 contrast with those of the Arthropoda and Vertebrata. And the 

 extreme cases of longevity, the immortal types, occur only among 

 the lower classes. Most of them are found among the Foramini- 

 fera; then follow the MoUuscoidea and Mollusca, and a few are also 

 met among the Crustaceans, but none among the other arthropods 

 and the vertebrates. 



2 Within each order and class, again the lower subclasses tend 

 to furnish the greater percentage of persistent forms. This is 

 best exemplified by the Pelecypoda (i6 per cent) and Gastropoda 

 (30 per cent) in contrast with the Cephalopoda (2 per cent), and 

 within the latter by the fact that the Nautiloidea contain all the 

 persistent types and the Ammonoidea and Dibranchiata none. Also 

 within the Crustacea, the Ostracoda (26.5 per cent) and Cirripedia 

 (20 per cent) contrast with the Malacostraca (4.5 per cent). 

 Among the vertebrates the primitive Selachians contain nearly all 

 the persistent types. 



This would seem to verify the assertion of some authors (cf., 

 for example, Neumayr, Stdmme des Thierreichs, p. 106) that 

 groups that have been overtaken by their more advanced relatives 

 and descendants, become stagnant and either die out or continue 

 to exist in unvariable forms. These are the genera that trail in 

 their existence after the group when it has passed its climacteric 

 period. This feature is well illustrated by many diagrams seen in 

 works on fossils showing the range of divisions and having in 

 general the following f orm^ : 



^ As seen, for instance, in the diagrams given by Beecher for the ranges of 

 the orders of the trilohites in Zittel-Eastman, p. 638. Also Walcott, Cambrian 

 Brachiopoda, U. S. Geol. Siirv. Monogr. 1913, p. 316. 



