Table 23. --Vertical distribution of Diaptomus ashlandi on August 28 and 29, 1928 

 [Clear weather on both days] 



groups, one of relatively local geographical distribu- 

 tion and limited to the American continent and the 

 other of species occurring both in North America and 

 Europe. A third group consisting of the eurythermic 

 species Palaemonetes exilipes, Cottus ricei, and C. 

 cognatus does not belong to the glacial relict fauna as 

 defined by Ekman (1927) but is nevertheless of marine 

 origin. 



The first group, with very limited geographical 

 range and consisting in Lake Erie of Rhizosolenia , 

 Stephanodiscus, and Triglopsis, may very well be true 

 glacial "marine relicts" which could have entered Lake 

 Erie from the ocean under conditions described by 

 Wilson (1929). Rhizosolenia and Stephanodiscus are 

 common marine genera of American coastal waters and 

 T. thompsoni is closely related to the clrcumpolar 

 oceanic genus Oncocottus . 



To explain the presence of the more widely distrib - 

 uted European -American species of the second group in 

 American lakes by direct introduction from the sea 

 seems unwananted. Obviously they are of marine origin 

 and together with other cold-water stenothermic forms 

 may have extended their distribution as active or pas- 

 sive migrants through the agency of the glacial lakes. 

 They may have been either real or secondary glacial 



relicts, (the latter presupposing them to be distributed 

 from a freshwater point of introduction into North 

 America). In either case, if we are to accept the 

 evolutionary hypothesis that the same species has never 

 originated in two different places, the presence of 

 Mysis relicta , Pontoporeia affinis , etc, , on the Amer- 

 ican and European continents implies a common source 

 of origin. 



In attempting to explain the direct introduction 

 of European -American species into Lake Erie from the 

 sea, several fundamental objections are encountered. 

 The common point of origin must in this case have been 

 the ocean and as no significant morphological changes 

 have taken place within the species since their separation 

 from the common source, it is obvious that the ancestral 

 marine fcsm must have had exactly the same specific 

 characters. None of the American relict species occur 

 In the sea at the present time, however, and to account 

 for the complete extermination of several clrcumpolar 

 marine species in so short a period of time and their 

 retention in isolated lakes calls for considerable stretch- 

 ing of the imagination. 



h appears more probable that the transition from 

 the sea to freshwater and subsequent development of 

 the present species took place in one locality, possibly 



190 



