Agricultural Productivity in Relation to Population 
1,530 million hectares of temperate land. We must have 8,200 
million hectares in all, capable of giving a diet containing meat 
and dairy products on a North American scale to 45,000 
million people. 
Finally, let me mention our requirements of minerals. They 
are being reduced by technical improvements and our descen- 
dants will not have to worry about mineral fuels, since they 
will have all the energy they want in the form of nuclear power, 
solar batteries, and cars driven by batteries or hydrogen, if 
necessary. The only exception is aluminium, but we could 
produce this from ordinary clays if supplies of bauxite ran out. 
If we assume a world population of 45,000 million consuming 
minerals at the present North American rate per head, the 
supplies of these minerals available in the top 1,500 metres of 
the earth’s crust would keep them going for some multiple of 
10° years (10° in the case of aluminium). 
So it looks like being a long time before our descendants have 
to dig deeper into the earth—or search in other planets outside 
it—for minerals. 
Q* 35 
