310 CLARK: CORRELATION OF PHYLOGENETIC SPECIALIZATION 



of development than the other; and the one representing the 

 greater degree of speciahzation ajways differs from the more 

 primitive in the partial or complete suppression of some struc- 

 tural feature, indicating that phylogenetic progress in the crin- 

 oids has been along the line of progressive structural degener- 

 ation, resulting in a constantly increasing structural simplification. 



If in each contrasted pair we place under each of the two con- 

 trasted characters the crinoid families in which it is manifested 

 it is evident that, although in nearly every case the families will 

 be differently divided, an examination of the bathymetrical range 

 of all of the entries under the more speciaUzed characters taken 

 together, contrasted with that of all of the entries under the less 

 specialized characters taken together, will enable us to ascer- 

 tain with a greater or lesser degree of accuracy the relationship 

 between phylogenetic development and depth. 



The number of contrasted pairs in each of the divisions of the 

 crinoid body, and the number of separate items or entries in 

 each of the divisions — that is, the number of citations of families 

 under all the subheadings taken together — are as follows: 



Number of 

 Number of separate 



headings items 



Calyx 10 68 



Arms 9 63 



Column 7 42 



Pinnules 5 35 



Disk 5 28 



General 1 7 



Total 37 243 



Being composed of the greatest number of structural units — ■ 

 each a group of similar structural elements — all of which vary- 

 more or less independently, the calyx is naturally the most 

 changeable division of the crinoid body. 



The arms, through their differentiation into phylogenetically 

 very distinct distal and basal portions, and through the close 

 interdependence of the latter upon conditions in the calyx, form 

 the next most changeable division. 



