DISCUSSION 
imitate such resistance in a susceptible population by artificial 
means. We would still never be sure about the level of activity 
of human defence systems which would protect man against an 
attack by unknown pathogens, particularly of an extraterrestrial 
variety, but, following Dr. Lederberg’s train of thought, the 
term “germ-free man’? may become less of an abstraction. 
However, even then, if I had my way, I would implant 
man with a known concoction of living infective agents under 
controlled conditions rather than let him go germ-free into 
the world, which I cannot conceive will ever become “‘ germ- 
inec’?s 
As far as the future of mankind is concerned, I may have 
painted a slightly darker picture than I originally intended and 
this may have accounted for Professor Haldane’s comment. 
However, I have related my mild pessimism to the problem of 
human folly with its unlimited connotations. 
Huxley: A germ-free world is an ecological absurdity, just 
as a perpetual motion machine is a mechanical absurdity. 
Koprowski’s comment about eradication is extraordinarily 
important. In most cases it is just nonsense to talk of eradica- 
tion. We are coming up against that with these manufacturers 
of pesticides who say ‘‘Now we can eradicate such-and-such 
a pest’’. I am certain that it is not possible to eradicate any 
abundant pest, though it is quite easy to eradicate a non- 
abundant non-pest in trying to do so. 
Talking of ecology, I was incidentally much interested in 
Koprowski’s story about the bats spreading further north and 
west in America, and I wondered if this had anything to do with 
the secular improvement of climate in the past 50 years. 
I am sure that one can get a lot of valuable ideas from 
considering problems like cancer and ageing on a comparative 
biological basis. For example, small birds and mice of com- 
parable size have quite different patterns of life. They have 
very similar rates of development to maturity but the birds 
have a much greater total life-span. When mammals are 
treated with carcinogens there is a certain delay in the mani- 
festation of the cancer. The delay is short in small mammals 
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