74 SALMON I A. [SECOND DAY. 



want of proper nourishment, and by unfavourable 

 weather. Males and females likewise, confined 

 from each other, have their generative powers 

 impeded; and trout, grayling, and salmon, will not 

 deposit their ova except in running water ; so that by 

 keeping them in tanks, the period of their maturity 

 might be considerably altered, I have seen charr even, 

 which had been kept in confined water from September 

 till July; and so slow had been the progress of the 

 ova, that they appeared to be about this time fit for 

 exclusion, though, in the natural course of tilings, 

 they would have been ripe in the end of October of 

 the year before. By attending to and controlling all 

 these circumstances, I have no doubt many interesting 

 experiments might be made, as to the possibility of 

 modifying the varieties of the salmo, by impregnating 

 the ova of one species with the spermatic fluid of 

 another. With fishes of other genera the task would 

 be still more easy. Carp, perch, and pike, deposit 

 their ova in still water in spring and summer, when it 

 is supplied with air by the growth of vegetables : and 

 it is to the leaves of plants, which afford a continual 

 supply of oxygen to the water, that the impregnated 

 eggs usually adhere ; so that researches of this kind 

 might be conducted within doors in close vessels, filled 

 with plants, exposed to the sun. I have myself kept 

 minnows and sticklebacks alive for many months in 



