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THE AMERICAN NATURALIST [Vol. XLI 



shore conditions. In a large and somewhat vague way isotherms 

 and isothermobaths constitute the barriers of the sea. Many, 

 and in some groups, most of the pelagic species are wide-ranging, 

 found in most seas, through a greater or less range of temperature. 

 The pelagic fauna has thus a considerable cosmopolitan element 

 and part of the differences which result in the contrasted poverty 

 and richness of pelagic fauna are due to changes in the numbers 

 of individuals and in the proportionate representation of the 

 various components, as much as, or even more, than to restrictions 

 in the distribution of species. In so far as the species of any group 

 of related organisms establish themselves throughout a wide, 

 coincident or overlapping range, in like degree isolation becomes 

 problematical as a factor in the origin of new or preservation of 

 old species. 



Our knowledge of the horizontal and vertical distribution of 

 pelagic organisms is lamentably incomplete and partial, and no 

 less so of the Chsetognatha than of other groups. Fowler (:06) 

 calls attention to the fact that he finds no published record of a 

 single species of that group between 160° E and 80° W, nearly the 

 whole of the Pacific Ocean! Unfortunately no report was pub- 

 lished on the Chaetognatha of the Challenger Expedition and 

 the results of later surveys have not yet appeared. We find, 

 however, an excellent summary of the known distribution in 

 Fowler's (:06) report on the 'Siboga' collections, based largely 

 on his Biscayan investigations, Fowler (:05), and the wwk of 

 Doncaster (:03) on the jVIaldive and Laccadive fauna, of Aida 

 ('97) on that of Japanese waters, of Steinhaus ('0()) and Strodt- 

 mann ('92) on collections from the Atlantic, and of various re- 

 corders in the lists of the Conseil permanent pour V exploration 

 de la Mer, from the waters of Northern Europe. The data thus 

 assembled by one whose critical knowledge of the species has 

 enabled him to sift out svnonyms and eliminate probable errors, 

 are far from being adecjuate to give a complete or satisfactory pre- 

 sentation of the distribution of Ch^tognatha in the seas named. 

 They are, nevertheless, of sufficient fulness to afford a basis for 

 the consideration of the extent to which isolation of species prevails 

 in this typical pelagic group of organisms and to mark out clearly 

 the necessity for additional data on vertical distribution and 

 breeding seasons for a critical and final analysis of the problem. 



