Kendall. — Rainbow and Steelhead Trout 191 



dency to large scales, associated with the southern limit. Prob- 

 ably owing to lack of uniformity in the intermediate condi- 

 tions, the intergradation from those with a tendency to small 

 scales to those of large may not have been perfect. 



Partial segregations may occur within large aggregations, 

 caused by local thinning out of interbreeding individuals. 

 Such partial interruptions to complete interbreeding may be 

 due to indirect barriers. An indirect barrier may be some 

 condition or influence tending to concentrate the individuals 

 of any section in a certain locality, while there is no actual 

 obstruction to their passage from one section to another. 



It goes without saying that it could not have been until the 

 final recession of glacial conditions that marine trout were 

 able to enter and permanently occupy fresh waters in the 

 regions of glaciation. While perhaps some rivers of the most 

 southern limit of this zone were accessible, they later became 

 unfavorable to the existence of the trout. In some of the 

 more southern and therefore earlier accessible waters where 

 conditions remained more favorable, the fish have survived. In 

 such instances we should expect the larger scaled fish. As suc- 

 cessively more northern waters became accessible, they were 

 occupied by trout. Some at first accessible became later in- 

 accessible, thus entrapping the first trout to enter them. 



As the habitat zone with, of course, its subordinate zones 

 moved northward with the recession of the glacial conditions, 

 the occupants of the respective zones would enter the acces- 

 sible fresh water encountered. They might be few or many. 

 But inasmuch as all regions were not provided with accessible 

 fresh waters, the present faunas would represent only those 

 which were derived from the respective zone reaching it at 

 the time of accessibility. Some which were cut off from the 

 sea by impassable barriers to the ascent of fish would contain 

 first arrivals, while the lower part of the water course would 

 be frequented by later arrivals. And so on, until theoreti- 

 cally there should be some sort of an intergradation of the 



