14 ROBERT TRACY JACKSON ON ECHINI. 



tions in the plates are arranged as nearly as practicable in systematic sequence following the 

 classification here presented. 



I am under great obligations to my friend, Dr. Hubert Lyman Clark, for his help in veri- 

 fying references, and help with my manuscript and other matters during my absence from home. 

 Miss H. I. Cowdery rendered valuable service as a typewriter by her careful painstaking copy- 

 ing of the manuscript for the printer. 



The requisite research work and the preparation of this memoir have taken my close 

 attention in available time for some seven years. That it has taken so long has been a 

 source of regret for more than one reason, but careful work cannot be done in a hurry. I 

 adopt the title "Phylogeny of the Echini" for this memior, because the phylogenetic rela- 

 tions expressed by stages in development and by variation are considered throughout the 

 work. As I have shown previously in my "Phylogeny of the Pelecypoda" (1890), and also 

 in studies of Echini (1896), and of Plants (1899), I believe that by following the Hyatt 

 methods of a comparative study of young, adult, fossil, and living forms we can arrive at a 

 series of facts which justify us in accepting them as expressing an approximation to the real 

 genetic relationships of the forms under consideration. The genealogical relations of the 

 Echini, as understood from the results of my studies, are shown in tabular form on p. 209. 



