84t SALMONID<E. 



the designation by which this fish is most generally known in 

 Scotland. 



" The early or late hatching of the Salmon-spawn in the 

 river is no doubt in a great measure regulated by the tem- 

 perature which may prevail after its deposition. In severe 

 winters, when the temperature of the river for many weeks 

 barely exceeds the freezing point, the ova remain in the 

 gravel at the bottom of the stream during that period with 

 the living principle comparatively suspended, until the more 

 genial temperature of the spring brings that principle into 

 more active operation. In the course of experiments made 

 in the beginning of 1838, I had an opportunity of observing 

 the different effects of temperature in facilitating or retarding 

 the development of the Salmon-spawn. In ova placed in a 

 stream of spring water, the average temperature of which was 

 40, the embryo fish was visible to the naked eye by the end 

 of the sixtieth day, and was hatched on the hundred and 

 eighth day after impregnation. That which the same parent 

 deposited the same day in the river, the average temperature 

 of which during the eight following weeks did not exceed 

 33, was not visible to the naked eye until the ninetieth day, 

 and was not hatched until the 10th of May, that is, one 

 hundred and thirty-one days after impregnation. The tem- 

 perature of the river, however, during the last forty days of 

 that period, had considerably increased, and on the day on 

 which the fishes were hatched, it had attained an elevation of 

 60. Were it, then, the fact that the young Salmon migrate 

 to the sea the same season they are hatched, the effects of a 

 mild or a rigid winter would alone regulate the period of 

 their departure from the river. This, however, is not the 

 fact, as the main body of the Salmon-fry regularly quit our 

 rivers about the first or second week in May, whatever may 

 have been the temperature of the previous winter, and in 

 this particular instance they were actually descending the 



