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This rather tedious argument has two corollaries of immediate
eugenic importance. The first is this. The more similar are the
environments of two human samples, the more likely are the observ-
able differences between the samples to be inheritable. The opposite
is also true in theory, that the more similar are their genetic con-
stitutions, the more likely are any differences to be environmental and
non-inheritable; but in view of our ignorance of the precise genetic
constitution of human populations, this has little applicability save in
special cases like that of identical twins.
When on the other hand there are obvious differences in environ-
ment between two groups, there is a strong presumption that many
of the differences between them will turn out to be mere modifications,
which would disappear if the environmental conditions were equal-
ized. This is not, of course, to say that the groups will not differ
genetically also: merely that the observed differences in characters
are not likely to be wholly inherited.
Genetics can provide interesting examples in which certain condi-
tions of environment may wholly mask the effect of a gene. The
classical case is that of Primula sinerisis. In this plant there is a white-
flowered variety and a red-flowered variety, which differ in regard to
a single Mendelian gene. The white remains white at all tempera-
tures ; but the red variety when raised at a high temperature produces
white flowers. A hot-house will thus entirely mask the perfectly real
genetic difference between the two.
Even more significant for our purpose is the case of the mutant of
the fruit-fly Drosophila known as abnormal abdomen, which depends on
a single recessive gene. Flies characteristic of this strain show a
bloated and rather abnormal-looking abdomen, with an extremely
poor and irregular development of the normal pattern of black bands.
However, all gradations from this to normal appearance are found.
Analysis has shown that in moist conditions the character manifests
itself fully, while in very dry conditions it does not show at all, and
the flies resemble the normal wild type. Environment may thus
wholly mask the effect of a pathological gene.
These cases introduce us to the further principle, somewhat para-
doxical at first sight, that equalizing the environment may either
increase or decrease the amount of visible variation in a group. In a
universe containing both dry and moist conditions, a mixture of wild-
type and abnormal-abdomen strains of fruit-fly would show a certain
range of variation. Equalize the environment by making the universe
wholly dry, and the population becomes uniform: but equalize
it by making the universe wholly moist, and the variability is