THE AQUATIC VERTEBRATES 1051 



vive •when thawed out. The adjustment in this respect probably 

 increases as one goes northward. Turner says of the Alaskan 

 Dallia: "When taken from the traps the fish are immediately put 

 into these baskets and taken to the village, where the baskets of 

 fish are placed on stages out of the way of dogs. The mass of fish 

 in each basket is frozen in a few minutes, and when required to 

 take them out they have to be chopped out with an axe or beaten 

 with a club to divide them into pieces of sufficient size to feed the 

 dogs. 



"The vitality of these fish is astonishing. They will remain in 

 those grass baskets for weeks, and when brought into the house 

 and thawed out they wiU be as lively as ever. The pieces which 

 are thrown to the ravenous dogs are eagerly swallowed, the ani- 

 mal heat of the dog's stomach thaws the fish out, whereupon its 

 movements cause the dog to vomit it up alive." 



The lower temperature limit is set to fish life by the freezing 

 point of the medium, 32° F. for fresh water, below this for salt 

 water. The upper observed Hmit in ponds is somewhere near 

 100 degrees.^ If the water is suddenly raised to this point, fishes 

 survive but a few seconds. While the upper hmit may be set by 

 the effect of the increased heat on the protoplasm, its effect may 

 be indirect and operate through the reduction in the amount of 

 held in suspension by the warm as compared with the cold water. 

 That fishes will attempt any temperature is evidenced by the 

 fact that they occasionally enter water in the National Park hot 

 enough to boil them. 



The adaptability of fishes to different temperatures is well shown 

 by Rhinichthys dulcis which is foimd in the streams coming from 

 the warm springs at Banff in the Canadian National Park, and 

 also in the icy waters of Vermilion Creek at the same place. The 

 same individuals are adjustable within wide limits, and the same 

 species is sometimes found over a long north and south range. 

 Nevertheless, temperature has doubtless played an important 

 part in setting a northern hmit to the migration of species, as 

 they followed the retreating ice of the glacial period. In North 



' Jordan and Richardson (Proc. U. S. Nat. Mus., 33 : 319-321) record Lucania browni 

 from a hot spring with a temperature of 128° F. 



