154 W. K. BROOKS. 



The dogma has been a most useful and suggestive working 

 hypothesis when well controlled, but when uncontrolled it has led 

 to the most fantastic and grotesque unscientific speculation. The 

 climax of inconsistency into which its blind adherents have been 

 led was well shown by the simultaneous appearance, in a recent 

 morphological journal, of two memoirs, one an essay on " The 

 Origin of the Vertebrates from the Arachnids," and the other on 

 " The Origin of Vertebrates from a Crustacean-like Ancestor." 



After my first examination of the second of these memoirs I 

 laid it down, much distressed in mind by the thought that this 

 author had unkindly descended from the sphere of experimental 

 research in physiology, to expose the unscientific methods of the 

 morphologists by a severe and well merited, if somewhat ponder- 

 ous, satire. 



In my next chapter on the morphological significance of appendi- 

 cularia I shall try to show that there is no philosophical necessity 

 for a phylogenetic explanation of duplicated structures in animals, 

 whether they are radical, bilateral, metameric or indefinite, and I 

 must refer the reader to that chapter for my reasons for including 

 append icularia, the copepod^' and the veiled medusae among the 

 primitive pelagic animals. 



The Phylogeny of the Metazoa. 



The primitive pelagic fauna, before the influence of the bottom 

 and of the shore had been brought to bear upon it, consisted of 

 small animals of simple structure; but we are forced, by the facts 

 of comparative anatomy and embryology, to believe that a number 

 of distinct types of structure were found among them. 



Most of the great metazoic stems show by their embryology that 

 they run back to simple and minute pelagic ancestors, and that their 

 common meeting-point must be projected back to a still more re- 

 mote time, before the diiferentiation of their pelagic ancestors had 

 been effected. After we have traced each great line of metazoa as 

 far back as we can from the study of fossils and by the aid of 

 comparative morphology, we still find these lines distinctly laid 

 down. The lower cambrian Crustacea, for example, are as distinct 

 from the lower cambrian echinoderms or pteropods or brachiopods 



