SALPA IN RELATION TO EVOLUTION OF LIFE. 153 



representative of the primitive Crustacea, and that the whole 

 history of the copepods has been pelagic; and that the veiled 

 medusae have been evolved in direct relation to pelagic influences ; 

 no one who makes these statements can hope to escape the charge that 

 his view " ist die unwahrscheinlichste von Unwahrscheinlichkeiten." 



The books all tell us that the free active appendicularia is the 

 " degenerated " descendant of an ancestor which crept over the 

 bottom ; that the nauplius is a secondary larval form ; that the 

 active free-swimming copepods are degenerated phyllopods ; and 

 that the locomotor hydro-medusa is, in its origin, a specialized 

 member of a sessile, polymorphic, hydroid cormus. 



The first of these opinions, tliat appendicularia is a degenerated 

 form, rests upon a supposed necessity for deriving the body of a 

 vertebrate, which consists of a series of segments homologous with 

 each other, but highly differentiated among themselves, from an 

 ancestral aggregation 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 corresponding parts of different animals, as the 

 result of phylogenetic inheritance ; or, to state it in a different way, 

 that the vegetativ^e duplication of parts iu 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 dis- 

 covery 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. 



