VARIETIES OF TROUT. 83 



eggs as are not supplied with water saturated 

 with air are unproductive. The experimenter 

 must be guided exactly by the instinct of the 

 parent fishes, who take care to deposit the im- 

 pregnated eggs that are to produce their off- 

 spring only in sources continually abounding 

 in fresh and aerated water. 



PHYS. But as every species of fish has a 

 particular and usually different time for spawn- 

 ing, I do not see how it could be contrived to 

 cross their breeds, or how the ova of a trout, 

 which spawns in December, could be impreg- 

 nated by the seminal fluid of the grayling, 

 which spawns in May ; for I conclude it would 

 be impossible to preserve the eggs of a fish out 

 of the body in a state in which they could 

 retain or recover their vitality. 



HAL. I believe I mentioned before, that I 

 had found instances, in which the ova of fish 

 were developed at a different period from their 

 natural one ; and I have no doubt, that a little 

 inquiry respecting the habits of fishes would 

 enable us to acquire a knowledge of the cir- 

 cumstances which either hasten or retard their 

 maturity. Plenty of food and a genial season 

 G 2 



