" The very positive statements which
I have italicised would lead most readers to believe that the alleged
fact had been demonstrated by a careful working out of the process in
some definite supposed cases. This, however, has nowhere been done in
Mr. Romanes' paper; and as it is _the_ vital theoretical point on which
any possible value of the new theory rests, and as it appears so opposed
to the self-destructive effects of simple infertility, which we have
already demonstrated when it occurs between the intermingled portion of
two varieties, it must be carefully examined. In doing so, I will
suppose that the required variation is not of "rare occurrence," but of
considerable amount, and that it appears afresh each year to about the
same extent, thus giving the theory every possible advantage.
Let us then suppose that a given species consists of 100,000 individuals
of each sex, with only the usual amount of fluctuating external
variability. Let a physiological variation arise, so that 10 per cent of
the whole number--10,000 individuals of each sex--while remaining
fertile _inter se_ become quite sterile with the remaining 90,000.
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