FOOD FROM THE SEA. 383 
on many of the most fundamental points are very great. Until more 
adequate means can be secured for carrying out actual experimental 
work at sea progress cannot possibly be rapid, and many of the problems 
cannot be studied in a satisfactory way. 
Slowly, however, we are building up a true science of the fisheries. 
The direction and velocity of the currents, the differences in tempera- 
ture and salinity of the water, and the variations in these factors from 
season to season and from year to year upon which the fluctuations in 
the abundance of fish must very largely depend, are being gradually 
worked out and understood. The effects of wind and weather and of the 
varving amount of sunshine falling on the water in different years are 
questions which are being studied. Then again, the natural history 
of the fishes themselves is the subject of much research ; their habits 
and food, when and where they spawn, the characters of the larval 
fishes and when and where their younger stages are to be found, all fall 
under this head. 
Many of the results of investigations on these lines have already from 
time to time been described and discussed at the meetings of our institu- 
tion, and for that reason I will not to-night dwell upon them, important 
indeed, essential—as they are for an adequate understanding of the 
problems which have been put forward. It is to a more general aspect 
of the matter that I would especially refer —the question of how the 
primary or fundamental food supply of the sea is built up. Suppose 
that during the late summer or autumn we capture in the waters of 
Plymouth Sound, a mackerel. The body of that mackerel represents 
some 8 ounces of excellent human food. What is the ultimate source 
from which that food has been produced? If we examine the stomach 
of the mackerel, we shall probably find it filled with small fishes, chiefly 
sprats and quite young herrings, with perhaps a certain number of those 
small, shrimp-like creatures which are known as Copepods. The young 
herrings and sprats have themselves fed largely upon Copepods of a similar 
kind. We conclude, therefore, that the body of the mackerel has been 
formed either directly, or indirectly, through the young herrings and 
sprats, from the organic substance contained in the bodies of the shrimp- 
like Copepods. Then if we carry the problem a stage further back and 
enquire how the bodies of the Copepods have been built up, we find with 
the aid of a microscope that their food consists in large measure of minute 
plants, chiefly belonging to the class of diatoms. 
We have here a particular and perhaps exceptionally simple and 
straightforward example of the general principle, applicable equally to 
land and sea animals, that the organic substance which constitutes their 
flesh is always derived either directly or indirectly from vegetable life. 
