582 proceedings: BIOL,qGICAL SOCIETY 



groups, Pacific and Atlantic, with no possible means of intercommuni- 

 cation. With the closing of the Arctic-Pacific gateway, two indepen- 

 dent lines of development began. 



The original ancestral forms doubtless occupied a northern zone, 

 the southern limit of which was a temperature barrier. The advancing 

 glacial conditions pushed the zone southward and formed a northern 

 border -barrier beyond which no aquatic animal could pass. 



The evolution of the environment was accompanied by evolution of 

 the occupant, with the very evident result that there now exist groups 

 of fishes adapted to different environmental conditions. Authorities 

 have indicated that the different environmental conditions are most 

 clearly defined by range of temperature. The different groups of Sal- 

 monids are most clearly determined by range in number of scales and 

 vertebrae. 



The Pacific vSalmonidae, with the exception of the chars, which are 

 probably of Atlantic origin, are sharply defined from the Atlantic Sal- 

 monidae by cranial characters. The changing environmental condi- 

 tions and the indirect barrier of distance, which had preceded the 

 Pacific- Arctic separation, had effected a partial segregation and modi- 

 fication of the ancestral form, which the previously mentioned land 

 barrier and the glacial period carried on to the results manifested by 

 present distribution of more or less differentiated forms. 



It is a well known fact that, as a rule, northern fishes are character- 

 ized by smaller scales and more numerous vertebrae than those of the 

 south. 



The present conditions necessary to the existence of the trouts indi- 

 cate that the trouts were evolved in and synchronously with the changes 

 of environmental conditions, culminating in those of the present time. 

 As the environmental zone and its subordinate zones moved northward 

 with the recession of the glacial conditions, the occupants of the re- 

 spective subordinate zones entered accessible fresh waters. 



It could not have been until the recession of the glacial conditions 

 that the marine trout were able to permanently occupy inland waters, 

 so as northern waters became accessible they were occupied by trout. 

 Inasmuch, however, as all regions were not provided with accessible 

 fresh waters, the present faunas represent only those which were de- 

 rived from the respective subordinate zones reaching the outlet of the 

 inland region at the time of accessibility. Such outlets may have been 

 accessible to one or two zones, and not to remaining zones. The trout 

 of present inland isolated waters indicate by their structure from which 

 zones they were populated and by what routes they probably reached 

 these waters. (Author's abstract.) The paper was illustrated by maps, 

 diagrams, and photographs of the trouts discussed. 



611TH MEETING 



The 6 nth regular meeting was held April 17, 1920, at 8 p.m., in the 

 lecture room of the Cosmos Club. Dr. A. D. Hopkins called the meet- 

 ing to order with 80 persons present. The minutes of the 6ioth meet- 



