VERTICAL DISTRIBUTION OF THE CHiETOG- 

 NATHA OF THE SAN DIEGO REGION IN 

 RELATION TO THE QUESTION OF 

 ISOLATION VS. COINCIDENCE 



ELLIS L. MICHAEL 



Scripps Institution for Biological Research of the University 

 of California, La Jolla, Cal. 



Introduction 



Ever since Jordan ( '05) called attention to the almost 

 universal neglect of Moritz Wagner's contention that 

 geographical isolation is an important factor in the for- 

 mation of species, " Jordan's Law" ('05, p. 547) that 

 "given any species in any region, the nearest related 

 species is not likely to be found in the same region nor 

 in a remote region, but in a neighboring district sepa- 

 rated from the first by a barrier of some sort," has been 

 subject to much controversy and diversity of opinion. 

 The conclusions of those biologists dealing with land 

 fauna have, as a rule, emphasized the fact of isolation, 

 whereas those of the marine biologist have tended to 

 emphasize the fact of coincidence, or at least to doubt 

 the truth of isolation. It is therefore part of the busi- 

 ness of the marine biologist, through whose investiga- 

 tions new and important data have been accumulated, to 

 throw as much light as possible upon the problems of 

 isolation and coincidence. This is particularly true with 

 regard to data concerning the Chsetognatha because, as 

 pointed out by Kofoid ('07), the group is exclusively 

 marine and pelagic, and so completely circumscribed as 

 to make it probable that their entire evolution has taken 

 place within the confines of the open sea. 



At the outset, the fundamental differences in the prob- 

 lem with reference to land and marine fauna must be 

 emphasized. Kofoid ('07, p. 241) has pointed out that 

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