128 THE LIFE OF THE SALMON 
do the salmon in the warmer Atlantic. The theory 
sounds plausible, but in accepting it one has to pre- 
sume that the salmon prefers warm water to cold. 
The more crude idea that early rivers are those 
which flow out of large lochs is an offshoot of the 
same theory, and may be dismissed at once by merely 
noticing how many early rivers do not come out of 
large lochs, and that practically all the late rivers. 
of the West Highlands do. A third idea which has 
been developed at considerable length, more especi- 
ally in magazine articles, is that the ascent from sea 
to river takes place when the temperatures of the 
salt and fresh water approximate. We have there- 
fore the physiologist stating that the condition of 
nutrition determines the “return” from the sea, 
and the physicist stating that the influence of 
temperature alone determines the same movement. 
I have already referred to the question of nutrition, 
and have shown that in my view it is not a complete 
explanation of the habit of ascent from sea water. 
In examining the theories as to temperature it is re- 
markable how much of theory and how little of fact 
is to be found. Only in arguments as to the ap- 
proximation of sea and river temperatures do we find 
a genuine attempt to deduce from thermometric 
readings. When such readings are taken in sufficient 
numbers to show the real conditions which usually 
obtain, it becomes at once evident that the idea that. 
salmon are drawn, as it were, from a cold sea by and 
to a warm river must be given up. The facts show 
that at the time when early fish are running all our 
rivers are much colder than the sea on either coast. 
