CLARK: CORRELATION OF PHYLOGENETIC SPECIALIZATION 311 



The column, in spite of being fundamentally a simple linear 

 series of similar ossicles, shows very great diversity; and it is 

 chiefly as a result of this diversity in the column and the conse- 

 quent necessity for a compensating mechanical readjustment 

 that the calyx exhibits such a great amount of variation. 



The pinnules and the disk, owing to their intimate connec- 

 tion with the gathering of the minute organisms which serve as 

 food, are able to vary but little from a fixed optimum type. 



If we take all of the entries under all of the more primitive 

 characters (1) in all of the thirty-seven contrasting pairs, and 

 similarly all of the entries under all of the more specialized 

 characters (2), and determine their frequency at different depths, 

 we get the following table. Under the heading ''General" is 

 included only the number of component ossicles in the skeleton, 

 which decreases greatly more or less in correlation with phylo- 

 genetic progress along other lines. 



TABLE 1 



