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Ch. XL.] 



MIGEATION OF SPECIES. 



371 



All are aware tliat tliere are certain fisli of passaj^-e wliicli 

 liave tlieir periodical niig-rations, like 



some 



The salmon, towards the season of spawning, ascends the 

 rivers for hundreds of miles, leaping up the cataracts which 

 it meets in its course, and then retreats again into the depths 

 of the ocean. The herring and the haddock, after frequentino- 

 certain shores, in vast shoals, for a series of years, desert 

 them again, and resort to other stations, followed by the 

 species which prey on them. Eels are said to descend into 

 the sea for the purpose of producing their young, which are 



seen returning into the fresh water 



myr 



mountin 



obstacle which occurs in the course of a river, by applyino- 



slimy 



climbin 



Wener 



largest inland lake in Sweden, which discharges its waters 

 by the celebrated cataracts of Trolhattan. But accordino- to 

 Professor Nilsson, when a canal was opened uniting the 



river 



Gotha with the lake by a series of nine locks, eels were ob- 

 served in abundance in the lake. It appears, therefore, that 

 though they were unable to ascend the falls, they made their 

 way by the locks, by which in a very short space a difference 

 of level of 114 feet is overcome. 



Gmelin says, that the Anseres (wild geese, ducks, and 

 others) subsist, in their migrations, on the spawn df fish • 

 and that oftentimes, when they void the spawn, two or three 

 days afterwards, the eggs retain their vitality unimpaired.! 

 When there are many disconnected freshwater lakes in a 



deemed 



remote 



become stocked with fish from one common 



eggs of these 



but it has been suggested, that the minute 

 animals may sometimes' be entangled in the^ feathers of 

 waterfowl. These, when they alight to wash and plume 

 themselves in the water, may often unconsciously contribute 

 to propagate swarms of fish, which, in due season, will supply 



them 



Some of the water-beetles, also 



as the 



* Phil. Trans, 1747, p. 395. 



B B 2 



t Amocn. Acad., Essay 75 



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