164 FISH-CULTURAL ASSOCIATION. 
NATURAL CAUSES INFLUENCING THE’ MOVEMENTS 
QE FISH IN RIV ERS: 
BY MARSHALL MC’DONALD. 
If we will consider for a moment the varieties of conditions 
that concur in and modify agricultural production, we will be 
better prepared to appreciate the multiple influences that enter 
into the question of maintaining and increasing the production 
of our fisheries. . 
The farmer of to-day has a guide in the conduct of the prac- 
tical operations of agriculture, the collective experience of all 
who have preceded him. The observations of many generations 
condensed in proverb and apothegm, and handed down from 
father to son, gives to the unlettered peasant the interpretation 
of natural signs, the forecast of seasons and the empirical rules 
by which he tills and sows and garners the unequal harvests, 
which the unequal seasons bring. 
Less than a century ago, chemistry allying herself with agri- 
culture, laid the foundation of rational methods, and since then 
chemists and botanists, physicists and physiologists, have been 
busy with their investigations, each contributing in some essen- 
tial particular to the solution of the important problem of in- 
creasing and maintaining the fertility of the soil. 
In those countries, like England for example, where the re- 
sults of scientific investigations have been formulated into rules 
of practice, the average production of cereals per acre now ex- 
ceeds two-fold, and often three-fold, the average production per 
acre two hundred years ago. 
This result has been accomplished in the face of an intensive 
system of cropping, which long ago would have rendered the 
fertile fields of England unproductive moorlands, or barren 
wastes, but for the lessons taught by chemists in its application 
to agriculture, and appropriated and applied in practice. 
Just in proportion as man has learned to dominate the condi- 
tions which influence agricultural production, he has been en- 
abled to raise the average yield per acre; but, unequalities of 
