182 
SAT, MON. 
be for a time to its nature, in no long time for the most part 
it is again impelled to press its course up the stream in spite 
of hindrances; and yet again a change takes place, and the 
same individual is equally eager to retrace its course. ihat 
there is a necessity for these alterations of action is seen by 
appearances of the fish which are subject to them; but m their 
nature they are too obscure to be comprehended by us; although 
we become aware of the existence of causes which hasten or 
delay them in the instance of individuals. 
It was an early opinion, which is also supported by the 
authority of Eondelctius, that the spawn of the Salmon was 
shed and hatched in the sea; but late experiment has proved 
that the presence of salt-water is fatal to the development into 
life of the fertilizing property of the milt, as also of the 
impregnated egg if it come in contact with it. It is also shewn 
in the “Intellectual Observer,” (April, 1864,) by tlie Eev. M. 
J. Berkeley, that in a sluggish flow of fresh water or in a tank 
the grains of roe are exposed to the danger of becoming covered 
with a parasitie growth of vegetable fibres of several species 
which deprive them of the power of further development; and 
this important observation may assist us in accounting for some 
of the mishaps which have attended the attempts that have been 
made to bring to life and educate the young fish which have 
been taken from their native beds, and placed in tanks or 
artificial beds. The presence of slime or mud is hurtful to the 
egg or newly-developed young; and it is the opinion of observers 
that the grains are injured if they lie in the gravel so near 
as to touch each other. 
But when escaped through these various dangers much 
difference of opinion has been expressed in reference to Ae 
leimth of time during which the grains may remain buried 
in °the gravel, and consequently how long it is before they 
manifest the active powers of their existence. Nilsson says 
that in Sweden this is an hundred and thirty days, and in 
Scotland it has varied from an hundred to an hundred and 
forty; but so considerable a difference as this may fairly be 
iudged to be connected with the warmth or coldness of the 
climate and season; and from this we are. at liberty to conclude 
further what is also shewn by observation, that in still warmer 
districts, especially from the south and farthest west of 
