the geometrical phenomena of development with the
chemical. The geometrical symmetry of living things is the key to a
knowledge of their regularity, and the forces which cause it. In the
symmetry of the dividing cell the basis of that resemblance we call
Heredity is contained. To imitate the morphological phenomena of life
we have to devise a system which can divide. It must be able to
divide, and to segment as--grossly--a vibrating plate or rod does, or
as an icicle can do as it becomes ribbed in a continuous stream of
water; but with this distinction, that the distribution of chemical
differences and properties must simultaneously be decided and disposed
in orderly relation to the pattern of the segmentation. Even if a
model which would do this could be constructed it might prove to be a
useful beginning.

This may be looking too far ahead. If we had to choose some one piece
of more proximate knowledge which we would more especially like to
acquire, I suppose we should ask for the secret of interracial
sterility. Nothing has yet been discovered to remove the grave
difficulty, by which Huxley in particular was so much oppressed, that
among the many varieties produced under domestication--which we all
regard as analogous to the species seen in nature--no clear case of
interracial sterility has been demonstrated. The phenomenon is
probably the only one to which the domesticated products seem to
afford no parallel. No solution of the difficulty can be offered which
has positive value, but it is perhaps worth considering the facts in
the light of modern ideas. It should be observed that we are not
discussing incompatibility of two species to produce offspring (a
totally distinct phenomenon), but the sterility of the offspring which
many of them do produce.

When two species, both perfectly fertile severally, produce on crossing a
sterile progeny, there is a presumption that the sterility is due to the
development in the hybrid of some substance which can only be formed by the
meeting of two complementary factors. That some such account is correct in
essence may be inferred from the well-known observation that if the hybrid
is not totally sterile but only partially so, and thus is able to form some
good germ-cells which develop into new individuals, the sterility of these
daughter-individuals is sensibly reduced or may be entirely absent. The
fertility once re-established, the sterility does not return in the later
proge