CH. IX] THE PRODUCTIVITY OF THE SEA 193 



science than agriculture ; nevertheless there is some difficulty 

 in estimating the absolute quantity of organic matter, in the form 

 of crops, or flesh produced from cultivated land per unit area. One 

 turns naturally to the Annual Returns published by the Board 

 of Agriculture and Fisheries for this information, but only to be 

 disappointed. We learn, for instance, that the average crop of 

 the period 1896—1906, in Great Britain was 31 '22 bushels of 

 wheat per acre, 38*92 bushels of oats, 28*59 bushels of beans, 

 5'78 tons of potatoes and so on^; but beyond these unilluminating 

 figures the official returns disclose little of general interest. The 

 quantities of the various crops thus accounted for do not represent 

 the productivity of the land, for the straw of grain crops must 

 also be considered as well as the " crop residues " of roots, etc., and 

 the weeds also produced per unit area. Considering both grain 

 and straw we find (from other sources of information) that the 

 average yield in dry substance of British crops may be put as 

 4,183 lbs. of wheat per acre, 3,827 lbs. of barley, 3,987 lbs. of oats, 

 2,822 lbs. of meadow hay, 3,461 lbs. of beans, and so on-. But 

 information as to the absolute quantities of proteid, fat, carbo- 

 hydrate and ash, yielded by cultivated land under different 

 conditions is apparently difficult to obtain. There is, however, 

 a German estimate by Biebahn and Rodewald, which puts the 

 productivity of cultivated land as equal to 1,790 kilogrammes 

 of dry organic substance per hectare. 



The available data for our comparisons are : — 



(1) In terms of flesh : 



Cultivated land, 



83*5 kgs. of beef per hectare (Biebahn) ; 



Cultivated fresh-water carp-ponds (Germany), 



65*5 to 164 kgs. carp per hectare ; 



Cultivated mussel beds (Morecambe), 



8,000 kgs. mussel flesh per hectare ; 



Uncultivated cockle beds (Lancashire), 

 QQ kgs. cockle flesh per hectare^; 



1 " Agricultural returns," Bd. of Agric. and Fisheries. 

 - Warrington, Chemistry of the Farm, London, 1891. 

 3 "Flesh " is about 35 % of the total weight. 



J. F. 13 



