48 THE LIFE OF THE SALMON 
fish seems still uncertain. It is significant, more- 
over, that in these Siberian rivers the local species 
of Coregonus, which with us are purely fresh water 
fish (the vendace and powan), are fully migratory, 
as reported by Giinther. It is quite possible to 
imagine that in early times salmon were so numerous 
in our rivers that a suitable supply of food was 
impossible, and that therefore the habit of migrating 
to the sea developed, but this is a mere supposition. 
As a matter of natural history it is, on the other 
hand, I think, more in harmony with what is known 
of the habits of the other members of the group to 
suppose that the habit which has developed is that 
of migrating to fresh water primarily for the purpose 
of spawning, and that in large continental rivers 
and in rivers of land now disconnected from conti- 
nents the habit of remaining for long periods, and 
by some species of living more or less permanently, 
has been acquired. 
If, however, as I have shown, it happens that in 
some seasons in our islands salmon ascend our 
rivers in comparatively poor condition, a serious blow 
is struck at the view that the primary cause of 
ascent from the sea is the state of satiety. Along 
with the procuring of food, the other great instinct 
is that of reproducing the species, and a fish not 
up to the normal standard of high nutrition may 
still be impelled by this instinct, even if the repro- 
ductive elements are, it may be, unable to produce 
eight or nine hundred eggs per pound weight of fish. 
I have dealt with this point of the marine as 
opposed to the fresh water origin of Salmonide, 
