COLIN CLARK 
or a similar order of magnitude to our minimum food 
requirements. 
In any case, the plants which we grow for food produce only a 
certain proportion of their substance in the form of starches, 
sugars, proteins, etc., which we can eat. The rest, apart from 
water content, consists of fibres which we plough back into the 
soil, or feed to animals, though occasionally we can use them for 
textiles or pulp. 
The palaeolithic hunter required 10 square kilometres per 
person to feed himself; the neolithic herdsman, 0-1 square 
kilometre, or 10 hectares; the medieval peasant two thirds of a 
hectare of ploughland to produce cereals for subsistence, plus 
his woodland: the Indian rice grower one fifth of a hectare to 
produce subsistence; the Japanese one sixteenth of a hectare, or 
only 640 square metres. It is the growth of population, as we 
shall see below, which has provoked these agricultural improve- 
ments. But we are still far from the end of the road. 
Let us take a look into the distant future, at our descendants 
possibly living in much larger numbers on this earth, or even 
living in communities with carefully conserved supplies of 
oxygen and water, on inhospitable planets, or on natural or 
artificial satellites. If our descendants reach the numbers which 
are sometimes predicted for them, they are likely, in the process, 
also to attain levels of wealth and technical skill which would 
make such tasks as building artificial satellites and transferring 
colonies of people to them quite feasible. They would not, 
presumably, have to engage in hard physical labour. If we take 
the basic allowance of 250 kilograms for food and double it, to 
allow for some wood and wood pulp, and to provide a little 
variety for the diet, we have a requirement of 500 kilograms per 
person per year or 1,370 grams per person per day. We can 
also assume those improvements in chemistry which would 
enable them to use the whole dry weight of the plant, either as 
food or as fibre. How much space would be required to produce 
this amount of plant material, dry weight, under the best 
conditions? In others words, what are the best rates of photo- 
synthesis in grams per square metre per day, which have been 
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