PHYLOGENESIS IN CLASSIFICATION. 24! 



same functions in organisms, whether of near or distant ge- 

 netic relationship. 



Embryological Likeness of Organisms whose Mature Characters 

 are Diverse. In grouping the Mollusca with the Molluscoidea 

 it is not denied that they may differ in origin even that 

 in their earliest stages of development Brachiopods may be 

 akin to Worms and Echinoderms; and what animals are not? 

 The adult modes of life and construction of hard parts of 

 the Brachiopods presented greater resemblance to the Mol- 

 lusca in the Cambrian than they did to Worms or Echinoderms, 

 and it is not ignorance alone which has led the paleontolo- 

 gist to compare them in studying the faunas of geological 

 time. On the other hand, when we go back to the primi- 

 tive steps of development of the germ it is to be expected 

 not only that two branches will show likeness of develop- 

 ment, but if we should go back far enough we shall meet 

 with no visible distinction between the germs of all the 

 Metozoa; in fact all animals, if we go back far enough, may 

 be supposed to present no differences. On the ground of 

 embryology, the Tunicates are akin to the Vertebrates; the 

 Worms are akin to the Echinoderms, the Molluscoidea, and 

 the Vertebrates: but the differentiation took place very far 

 back in geological time. 



Evolution not Traceable between Different Classes. The ar- 

 rangement into branches, therefore, is from a structural point 

 of view highly artificial; and for purposes of tracing the his- 

 tory, or even from a taxonomic point of view, it is of little 

 importance to deal with characters more ancient or of higher 

 rank than the class characters. 



It may be convenient to associate the classes together into 

 larger groups; but to reach the point of real union of their 

 characters, in order to associate two or more classes in 3 

 common group, leads us far back into the uncertain mists of 

 the earliest geological time, and into the similar mists of em- 

 bryonic homogeneity. It is impracticable in the present 

 stage of science to trace the evolutional history of classes. 



The Mollusca and Molluscoidea are of particular interest 

 because, lacking internal skeletal parts, and developing a 

 single or two-valved shell, there is concentrated on this shell 



