384 B. J: ALLEN, 
The amount of animal life, therefore, which a given area of sea or land 
can support, depends upon the amount of plant life on which the animal 
life can feed. 
When we push the enquiry further back we find, as you know, that 
the bulk of the plant substance consists, in addition to water, of com- 
pounds of the element carbon, which are known under the general name 
of organic compounds. This carbon is derived entirely from the carbonic 
acid gas present in the air or dissolved in the water, the gas being split up 
and the carbon assimilated by the plant in the presence of sunlight. The 
source of the energy by means of which all organic matter is built up is 
the light of the sun, whilst the great mass of the solid substance contained 
in the bodies of both animals and plants is derived ultimately from 
carbonic acid gas, obtained in the case of land plants from the air, in the 
case of sea plants from gas dissolved in the water. In addition to carbon 
and water the plant requires a number of other substances, chiefly in- 
organic salts, but the quantities of these which are necessary are com- 
paratively small. In the case of land plants these salts are obtained from 
the soil, in the case of sea plants from the water in which they are dis- 
solved. 
It may be interesting here to compare the yield of organic substance 
derived from a given area of sea or fresh water with that from a similar 
area of land, to compare the harvest of the sea with the harvest of the 
land. 
It has been calculated by Brandt, from the catches of fishermen 1m 
an enclosed harbour, that the annual yield was 89 Ibs. of fish per acre. 
In Continental carp ponds, where the culture has been carefully carried 
on, 95 Ibs. of fish per acre per year have been obtained. 
Making a similar calculation for the North Sea from the statistics of 
fish landed, we get, as we should expect, a much lower annual yield, 
namely, 15 Ibs. of fish per acre. The average value of this is only 1s. 6d. 
per acre per year. 
Beds of shellfish give a very much higher yield, but they of course 
in reality, owing to the tidal currents which pass over them, draw their 
food supply from a much greater area of water than that of the sea-floor 
to which they are attached. Johnstone finds for the uncultivated mussel 
beds of Morecambe Bay, on the Lancashire coast, a yearly production of 
86 cwts. (or nearly 10,000 lbs.) per acre, valued at £14 16s. per acre. 
For comparison with these figures here is one taken from agricultural 
statistics. Young bullocks fed on cultivated land give an average annual 
yield of 73 Ibs. of beef per acre. 
Putting the figures side by side we have first the mussels from More- 
