DISCUSSION 
contraception would be ended. One wants a family of a partic- 
ular size for its own sake, regardless of the question of feeding 
it. This is a fundamental point which probably makes me more 
damnable in this view than if I were to accept contraception 
as an unfortunate necessity. Even if we could feed all the people, 
we would still need the ability to control reproduction. 
Parkes: If I understand Colin Clark correctly, he said that 
the cultivable land area of the earth would support 45,000 
million people. I wonder what that implies in terms of popu- 
lation density? 
Clark: Half an acre each, in the temperate lands, nobody in 
the polar or the desert regions, but there would be only one- 
sixth of an acre each in the tropics. Half an acre each means 
roughly 1,000 people per square mile. 
Pirie: ‘That is the population density of Hertfordshire now. 
Hoagland: I understand that Holland has goo people per 
square mile. 
Pirie: Bermuda has 2,000 people per square mile and people 
go there quite voluntarily! 
Clark: Yes. We will grow short of recreational space long 
before we run short of food. We are only just beginning in 
England and Holland to think more of our recreational space 
than of our agricultural space. It will be several centuries 
before we experience any major pressure on land. 
Lederberg: You underestimate the force of exponential 
growth! If there are going to be 6,000 million people by the 
end of this century and if there is logarithmic growth with a 
doubling time of 30 or 40 years, we shall reach your figure of 
45,000 million in a single century. 
Clark: At the world’s present rate of growth of 1-8 per cent 
a year it takes about 40 years to double the population. The 
15-fold increase to 45,000 million would take a century and a 
half, at this rate. 
Hoagland: ‘The arable land will be decreased rapidly because 
of this doubling of the population every 30 or 40 years. In 
that period we shall also have to double all our highways, 
double all our institutions and buildings, our towns and villages, 
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