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concerned, largely override constitutional influences, it is clear that
the social environment itself often exercises a selective influence which
may be of great importance.
This selective influence is of two distinct kinds, which we may call
pre-selective and post-selective. In simplest terms, pro-selective
influences are those which attract certain types into an environment
and discourage others. Post-selective influences are those which act
on the population subjected to the environment, favouring certain
evolutionary trends within it at the expense of others,
As a biological example, think of the assemblage of animals found
living in caves. They are characterized broadly by poor eyesight
and reliance on touch; the extreme types are eyeless, and pale or even
colourless. It seems clear that both pre- and post-selective processes
must have here been at work. Animals with somewhat poorly
developed eyes, which shun the light and normally live in dark corners,
will more frequently find themselves in caves, and will bo likely to
survive there better than more active and more "normal" types* But
once a cave-population is established, selection will be at work to
encourage the development of tactile and other organs for use in the
dark; it will also cease to operate strongly or at all on the genes
responsible for keeping up full pigmentation or perfect eyes, so that
these will in many cases degenerate.
A striking example is that concerning the selective influence of the
environment provided by fields of cultivated cereals. As Vavilov has
shown,1 this favoured certain other plants, which could then flourish
as what the farmer calls weeds, in association with the crop* Among
these weeds were wild grasses related to the cultivated cereal; and in
certain climatic conditions, these weeds flourished relative to the crop,
became the dominant species, and were then used by man as the
basis for a new crop-plant.
Just as cultivation of one crop-plant here provided the basis for the
later development of another, so the social environment appropriate
to one stage of human culture gives opportunities for the expression
of human traits which may be destined to become dominant at a later
stage. The eliciting effect of environment is in both cases essential
The United States furnishes a classical human example.    Pre-
selection was at work on the pioneers.   The human cargo of the
Mayflower was certainly not a random sample of the English popula-
tion.   Religious zeal, independence of character, perhaps a tendency
to fanaticism, together with courage, must have been above the
average among the leaders, and probably in the whole band*  The
1 Vavilovj 1926.