Scarcity of food may put
sufficient pressure on him to cause emigration, but rarely death. The
importance of Malthus' argument to eugenics is too slight to warrant
further discussion.
When the non-sustentative forms of lethal selection are considered, it
is seen very clearly that man is not exempt from the workings of this
law. A non-sustentative form of natural selection takes place through
the destruction of the individual by some adverse feature of the
environment, such as excessive cold, or bacteria; or by bodily
deficiency; and it is independent of mere food-supply. W. F. R. Weldon
showed by a long series of measurements, for example, that as the harbor
of Plymouth, England, kept getting muddier, the crabs which lived in it
kept getting narrower; those with the greatest frontal breadth filtered
the water entering their gills least effectively, and died.
But, it was objected, man is above all this. He has gained the control
of his own environment. The bloody hand of natural selection may fall on
crabs: but surely you would not have us think that Man, the Lord of
Creation, shares the same fate?
Biologists could hardly think otherwise. Statisticians were able to
supply the needed proof. A selective death-rate in man can not only be
demonstrated but it can be actually measured.
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