ilar, no such principle can
be demonstrated to determine sterility or fertility in general. For
example, though all our Finches can breed together, the hybrids are
all sterile. Of Ducks some species can breed together without
producing the slightest sterility; others have totally sterile
offspring, and so on. The hybrids between several _genera_ of Orchids
are perfectly fertile on the female side, and some on the male side
also, but the hybrids produced between the Turnip (_Brassica napus_)
and the Swede (_Brassica campestris_), which, according to our
estimates of affinity, should be nearly allied forms, are totally
sterile.[73] Lastly, it may be recalled that in sterility we are
almost certainly considering a meristic phenomenon. _Failure to
divide_ is, we may feel fairly sure, the immediate "cause" of the
sterility. Now, though we know very little about the heredity of
meristic differences, all that we do know points to the conclusion
that the less-divided is dominant to the more-divided, and we are thus
justified in supposing that there are factors which can arrest or
prevent cell-division. My conjecture therefore is that in the case of
sterility of cross-breds we see the effect produced by a complementary
pair of such factors. This and many similar problems are now open to
our analysis.

The question is sometimes asked, Do the new lights on Variation and
Heredity make the process of Evolution easier to understand? On the
whole the answer may be given that they do. There is some appearance
of loss of simplicity, but the gain is real. As was said above, the
time is not ripe for the discussion of the origin of species. With
faith in Evolution unshaken--if indeed the word faith can be used in
application to that which is certain--we look on the manner and
causation of adapted differentiation as still wholly mysterious. As
Samuel Butler so truly said: "To me it seems that the 'Origin of
Variation,' whatever it is, is the only true 'Origin of Species,'"[74]
and of that Origin not one of us knows anything. But given
Variation--and it is given: assuming further that the variations are
not guided into paths of adaptation--and both to the Darwinian and to
the modern school this hypothesis appears to be sound if unproven--an
evolution of species proceeding by definite steps is more, rather than
less, easy to imagine than an evolution proceeding by the accumulation
of indefinite and insensible steps. Those who have lost thems