ORIGIN OF SPECIES OF SALMONID^. 15 



structure of the various species indicate must have been during a time of comparatively 

 free intercommunication of oceans, and of comparatively uniform conditions in those 

 portions of all seas in which they lived. 



The ancestral salmonid may have occupied the Pacific, Arctic, and Atlantic Oceans, 

 or it may have been restricted to the Arctic. Changes which were evidently initiated as 

 early as the Miocene may have pushed some arctic ancestors southward into the Pacific, 

 if they did not already occur there. It is well established that in the Pliocene the Pacific 

 was occluded from the Arctic by land connections between Alaska and Siberia. The 

 salmonids were then actually segregated into two groups, Pacific and Atlantic, with no 

 possible means of intercommunication. With the closing of the Arctic-Pacific gateway, 

 two independent Unes of development began. 



The original ancestral forms doubtless occupied a sort of 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 and oscillation of the environment was accompanied by evolu- 

 tion of the occupants, with the very evident result that there now exist groups of fishes 

 adapted to different environmental conditions. 



The Pacific Salmonidae, with the exception of the chars which were probably of arctic- 

 Atlantic origin, are sharply defined from the Atlantic Salmonidae by cranial characters 

 (Regan 1914, p. 406-407). The changing environmental conditions and the indirect 

 barrier of distance, which preceded the Pacific-arctic separation, had effected a partial 

 segregation and modification of the ancestral form, which the previously mentioned 

 land-barrier and the glacial period carried on to the results manifested by present dis- 

 tribution of more or less differentiated forms. It is a well-known fact, that, as a rule, 

 northern fishes are characterized by smaller scales and more numerous vertebrae than 

 those of the south. 



The present conditions necessary to the existence of the salmonids indicate that they 

 were evolved in and synchronously, with changes of environmental conditions culmin- 

 ating in those of the present time. As the environmental zone and its subordinate marine 

 zones moved northward with the recession of the glacial conditions, the occupants of 

 the respective subordinate zones entered accessible fresh waters. It could not have been 

 until the final recession of the glacial conditions that the marine salmonids were able 

 permanently to occupy inland waters of the glaciated regions, so, as northern waters 

 became accessible, they were occupied by them. Inasmuch, however, as all regions were 

 not provided with accessible fresh waters, the present faunas represent only those which 

 were derived 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 the remaining zones. The salmonids of present inland isolated waters 

 indicate by their structure from which zones they were populated and by what routes 

 they probably reached those waters. Thus through physiological adaptation a complex 

 of environmental factors, seemingly paramount of which was that of temperature, 

 determined the distribution, differentiation, and certain so-called habits and behavior 

 of salmonid fishes. 



