WOOD, CRINOID ARMS IN STUDIES OP PHYLOGENY 17 



Conclusion 



The consideratious presented in the above paper constitute only a be- 

 ginning in a line of investigation which seems to promise good results, if 

 followed out in genera which have highly modified arms. From the 

 proximal to the distal portion of the arm, we find a series of changes in 

 structural features which succeed one another in a definite order. These 

 changes may be interpreted as stages in development, each individual re- 

 peating the stages present in its immediate ancestor and adding, in the 

 distal portion, new characters of its own until the number of characters 

 becomes too great for representation in the life history of a single organ- 

 ism, and certain characters, usually the earlier ones, are greatly abbre- 

 viated or are omitted from the ontogeny of highly modified descendants. 

 When thus intei'preted, the arms of crinoids furnish evidence from which 

 the phylogenetic relations of different species and genera can be inferred. 

 With the attention once drawn to the subject, it will probably be found 

 that the number of genera which may be studied by this method and the 

 degree of modification existing are greater than would appear at first 

 thought. 



