th useful characters. Mr. Romanes thinks
that they would persist, and urges that "whenever this one kind of
variation occurs _it cannot escape the preserving agency_ of
physiological selection. Hence, even if it be granted that the variation
which affects the reproductive system in this particular way is a
variation of comparatively rare occurrence, still, as _it must always be
preserved_ whenever it does occur, its influence in the manufacture of
specific types _must be cumulative_." 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. This
peculiarity is not correlated with any external differences of form or
colour, or with inherent peculiarities of likes or dislikes leading to
any choice as to the pairing of the two sets of individuals. We have now
to inquire, What would be the result?

Taking, first, the 10,000 pairs of the physiological or abnormal
variety, we find that each male of these might pair with any one of the
whole 100,000 of the opposite sex. If, therefore, there was nothing to
limit their choice to particular individuals of either variety, the
probabilities are that 9000 of them would pair with the opposite
variety, and only 1000 with their own variety--that is, that 9000 would
form sterile unions, and only _one_ thousand would form fertile unions.

Taking, next, the 90,000 normal individuals of either sex, we find, that
each ma