NO. IO PHYLOGENETIC STUDY OF RECENT CRINOIDS CLARK 3 



Thus in every large group a large number of such contrasted pairs 

 of characters must be employed, each of them having a more or less 

 limited value. 



A detailed study of the pairs of contrasted characters used in the 

 differentiation of the groups of recent crinoids, and especially of the 

 relation of the two components of each pair to each other, should 

 not only indicate the phylogenetic interrelationships of the various 

 types, but should also show clearly by what broad principle phylo- 

 genetic advance, or specialization, has come about. 



Therefore in addition to determining the correct phylogenetic 

 status of each of the groups of recent crinoids, an attempt will be 

 made in the present paper to analyze the pairs of contrasted charac- 

 ters in an effort to discover the significance of each of the compo- 

 nents, and thereby to indicate along what lines the phylogenetic 

 development of the crinoids has progressed. 



THE COURSE TAKEN BY PHYLOGENETIC PROGRESS, OR PRO- 

 GRESSIVE SPECIALIZATION, AMONG THE CRINOIDS 



The dominant feature of the progressive specialization among 

 the crinoids from the earliest times to the present day has always 

 been a process of progressive simplification in structure, the result 

 of a process of progressive atrophy or suppression affecting some 

 part or other of the organism. Thus the more specialized types 

 differ from the more generalized through the atrophy or suppression 

 of some important structural element, while the later groups are 

 differentiated among themselves according to the lines which this 

 atrophy or suppression has followed. 



In a broad way this has long been appreciated ; we recognize that 

 the (recent) Articulata are distinguished from the Inadunata by the 

 extreme atrophy of their calyx, involving in most cases the com- 

 plete disappearance of certain essential elements ; the comatulids 

 are differentiated from all other (recent) types by the suppression 

 of the column, excepting only the topmost columnal which becomes 

 permanently attached to the calyx ; Holopus is differentiated from 

 all other (recent) genera through the suppression of the column 

 excepting only the base, upon which directly the calyx rests ; the 

 Phrynocrinidae differ from the Bourgueticrinidae in the complete 

 suppression of the radicular cirri; and the Bourgueticrinidae differ 

 from the Phrynocrinidae in the suppression of the terminal stem plate. 

 But as yet no attempt has been made to apply this principle to all of 

 the differential characters which collectively make up the crinoid 

 whole. 



