J. B. S. HALDANE 
genetically we do not know what selective forces are acting on 
it. For example the selective value of ABO blood group mem- 
bership begins long before birth, and continues into middle life, 
where it is manifested by different frequencies of gastric and 
duodenal ulcers and other diseases. It is reasonably sure that 
the forces of selection acting on human beings have changed 
drastically in one or two generations, and will go on doing so. 
For the last ten thousand years or so, in fact since man ceased 
to be a rare animal, I think selection has been mainly for 
immunity for infectious diseases. No doubt this has kept the 
level of non-specific defences, such as the capacity for making 
gamma globulins, from deteriorating, but most of it has been 
futile or harmful. Various abnormal conditions, including 
several abnormal haemoglobins, thalassaemia, and glucose 
phosphate dehydrogenase deficiency, confer resistance to mala- 
ria at the price of ill-health or even death. The hundreds of 
millions of deaths by which the European stocks secured 
resistance to tuberculosis were not merely futile, for tuberculosis 
is now a rare and curable disease. They were almost certainly 
harmful. As Penrose! first pointed out, selection for resistance 
to specific diseases is probably selection for genes which 
were initially rare because they lowered fitness in the 
absence of the disease in question. With the abolition of 
the infectious diseases our descendants will gradually regain 
fitness. 
However, insofar as medical science enables people with 
congenital abnormalities who would formerly have died young 
to reproduce themselves, it is dysgenic, as has often been pointed 
out. The remedy for this is education. Once a man with rectal 
polyposis and a woman heterozygous for haemophilia realize 
that it would be wrong to have children, there is good reason 
why they should marry, using contraceptives, or after one or 
both have been sterilized. 
We do not know how selection is acting in economically 
advanced countries. Most people marry, and the main selec- 
tive agency is now fertility, not survival. We have little idea 
how much of the variance in fertility is genetically determined. 
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