Kendall. — Rainbow and Steelhead Trout 189 



reconcile this idea with a development of a small scaled form 

 from a coarse scaled form of Europe or Asia, which moved 

 successively southward and inland in very irregular and di- 

 verse routes, producing offshoots with both large and small 

 scales. To me, the Salmonidae afford a striking example of an 

 anciently established "zone system" which is now applicable to 

 the whole salmonid world. We know that on the Pacific 

 coast the genus Salmo occurs as a more or less marine fish 

 from the neighborhood of 57° to approximately 34°, north 

 latitude. The most southern inland trout occurs in Durango, 

 Mexico, probably in the headwaters of the Yaqui River or 

 some neighboring stream, the mouth of which is in about 

 27° north latitude, near Guaymas. In lower California, a 

 trout is found in the San Matir Mountains in about Z2>° north 

 latitude. 



It would appear that authorities would have us to suppose 

 that the trout have reached the cold waters of these high 

 altitudes by slowly traveling from cold waters of Alaska into 

 warmer waters of Mexico, thence into cold waters again. 

 Surely there is no discoverable law for distribution of this 

 kind. Where streams contain trout in their upper waters and 

 sea-run forms do not occur, we must assume, it seems to me, 

 that the coastal region is beyond the limit of suitable condi- 

 tions. We therefore appear to have a marine zone or coastal 

 area of more or less favorable conditions comprised between 

 the degrees of latitude mentioned (57° and 34°, approx- 

 imately), and an inland and altitude zone comprising irregular 

 areas of distribution, extending to at least 30° north latitude, 

 and if the mouth of the river is considered, even to 27° north 

 latitude. Assuming that 75° north latitude now represents 

 conditions similar to those of latitude 57° north, which is 

 stated to be the southern limit of a general ice cap on the 

 Pacific coast in the last glacial period, and taking the latitude 

 of Arroyo Grande, or Santa Maria River, California, as the 

 present corresponding southern limit of marine trout, the 



