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stitution to degrade itself. That man shares this tendency we can be
sure, not only from analogy but on the all-too-obvious evidence pro-
vided by the high incidence in " civilized " populations of defects, both
mental and physical, of genetic origin.
In wild animals and plants, this tendency is either reversed or at
least held in check by the operation of natural selection, which here
again proves itself to be, in R. A. Fisher's words, a mechanism capable
of generating high degrees of improbability. In domestic animals and
plants, the same result is achieved by our artificial selection. But in
civilized human communities of our present type, the elimination of
defect by natural selection is largely (though of course by no means
wholly) rendered inoperative by medicine, charity, and the social
services; while, as we have seen, there is no selection encouraging
favourable variations. The net result is that many deleterious muta-
tions can and do survive, and the tendency to degradation of the
germ-plasm can manifest itself.
To-day, thanks to the last fifteen years' work in pure science, we
can be sure of this alarming fact, whereas previously it was only a
vague surmise.1 Humanity will gradually destroy itself from within,
will decay in its very core and essence, if this slow but relentless pro-
cess is not checked. Here again, dealing with defectives in the
present system can be at best a palliative. We must be able to pick
out the genetically inferior stocks with more certainty, and we must
set in motion counter-forces making for faster reproduction of superior
stocks, if we are to reverse or even arrest the trend. And neither of
these, as we have seen, is possible without an alteration of social
Whether or not I have been asking you to accompany me too far
into the visionary future, I will end this essay with a very concrete
suggestion for the present, backed by a warning from the immediate
Twenty-five years ago, when I had just taken my degree, the field
of heredity was still a battle-field. The Mendelians and the Bio-
metricians were disputing for its possession, and in the heat of the
struggle little mercy was shown by either side to the other. In the last
dozen years or so, however, the apparent conflict of principle has been
shown not to exist, and now, thanks to the work of such men as R. A,
Fisher and J. B. S. Haldane, we realize that the two methods of
approach are complementary, and that certain important problems
can only be solved by their simultaneous employment.
The present position of eugenists appears to me to be closely parallel
1 MuUer,H.J., 1935'-