SALMON. 183 



England, a much shorter time will be sufficient for the 

 purpose. It is known accordingly that the roe of this fish 

 has been developed into life in these last-named districts 

 within the space of sixty days; and in proof of this hastening 

 influence of temperature Dr. Knox procured grains of roe 

 which had lain in the sand for a hundred and sixteen days, 

 but had not yet shown signs of becoming hatched; yet when 

 these were placed in a bottle, with their native water, and 

 brought into a warm room, the actions of life very soon 

 displayed themselves. It is to be observed, however, that 

 when thus hastened on, and the regular course artificially 

 interfered with by a sudden application of heat, or too high 

 a degree of it, the very young fry are much less likely to 

 survive it. Dr. Davy found a temperature above seventy 

 degrees, and up to eighty, to be certainly fatal to the eggs 

 of fishes of this family, as also of some other fishes. In Mr. 

 Shaw's experiments on the breeding of Salmon, when the 

 temperature of the stream he employed was at thirty-nine 

 degrees, and of the main river from which the breeding 

 Salmon were taken thirty-three, with that of the atmosphere 

 thirty-six, the embryo after fifty days was seen to be able to 

 move under its covering; and it escaped from the egg when 

 the temperature was at forty-four: but the whole of this 

 implies a far deeper degree of cold than is the average of 

 rivers at the same season in the south-west of England. Mr. 

 Shaw remarks that the "ova which for a time previous to 

 being hatched had been almost daily in my hands for inspection, 

 did not appear to suffer at all from being handled. When I 

 had occasion to inspect the ovum I placed it in the hollow 

 of my hand, covered with a few drops of water, where it 

 frequently remained a considerable time without suffering any 

 apparent injury." But afterwards he admits that it shewed 

 an increase of activity from the heat of his hand, and the 

 variation of temperature thus produced would probably lead 

 to injurious consequence, although the young at last seemed 

 capable of surviving longer than Dr. Knox supposed. This 

 last-named gentleman found sometimes that they had quitted 

 the gravel by the 1st. of April, but at other times it was not 

 for upwards of a fortnight after this; while on another occa- 

 sion Sir Francis Mackenzie made the experiment, on the 



