But the
analogy fails when we turn to the negative or weak effects of pollen
from one species on a distinct species; for although some substances
which are extremely dissimilar, for instance, carbon and chlorine, have
a very feeble affinity for each other, yet it cannot be said that the
weakness of the affinity depends in such cases on the extent to which
the substances differ. It is not known why a certain amount of
differentiation is necessary or favourable for the chemical affinity or
union of two substances, any more than for the fertilisation or union of
two organisms.
Mr. Herbert Spencer has discussed this whole subject at great length,
and after stating that all the forces throughout nature tend towards an
equilibrium, remarks, "that the need of this union of sperm-cell and
germ-ccell is the need for overthrowing this equilibrium and
re-establishing active molecular change in the detached germ--a result
which is probably effected by mixing the slightly-different
physiological units of slightly-different individuals." (12/16.
'Principles of Biology' volume 1 page 274 1864. In my 'Origin of
Species' published in 1859, I spoke of the good effects from slight
changes in the condition of life and from cross-fertilisation, and of
the evil effects from great changes in the conditions and from crossing
widely distinct forms (i.e., species), as a series of facts "connected
together by some common but unknown bond, which is essentially related
to the principle of life.
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