162 JOHNS HOPKINS UNIVERSITY MORPHOLOGICAL MONOGRAPHS. 



9 



but highly differentiated among themselves, from an ancestral aggrega- 

 tion of similar, but less differentiated, segments. 



The second opinion, that the copepods are degenerated and that the 

 nauplius is a secondary larval form, is the result of a supposed necessity 

 for explaining the segmentation of the arthropods in the same way, 

 while the third view has its origin in the belief that the polymorphic 

 members of a hydroid cormus must have arisen through specialization 

 and division of labor from an ancestral undifferentiated aggregation. 



These are a few, from among many, illustrations of the general 

 acceptance among morphologists of a dogma which, while it is often 

 refined and qualified until its character is almost lost, may be broadly 

 stated as a belief that the homology between different parts of the same 

 organism is always to be explained, like the homology between corres- 

 ponding parts of different animals, as the result of phylogenetic inherit- 

 ance ; or, to state it in a different way, that the vegetative duplication of 

 parts in animals has a phylogenetic significance, and implies descent 

 from a duplicated ancestor. 



The dogma is not the dictum of any one teacher, and it has grown 

 almost imperceptibly from its starting-point in the discovery that the 

 body of a metazoon is an aggregation of cells, each with an individuality 

 of its own, specialized and differentiated by polymorphism and division of 

 labor, and each one homologous with an unicellular organism. 



The dogma has been a most useful and suggestive working hypoth- 

 esis when well controlled, but when uncontrolled it has led to the most 

 fantastic and grotesque unscientific speculation. The climax of incon- 

 sistency 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 Crus- 

 tacean-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 physi- 

 ology, to expose the unscientific methods of the morphologists by a 

 severe and well merited, if somewhat ponderous, 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 radial, bilateral, metameric or indefinite, and I must refer the 



