e human race is
a witness of an exceptionally unchanging and stable condition of things, if
the calculations of Mr. Croll are valid as to how far variations in the
eccentricity in the earth's orbit together with the precession of the
equinoxes have produced changes in climate. Mr. Wallace has pointed
out[229] that the last 60,000 years having been exceptionally unchanging as
regards these conditions, specific evolution may have been {227}
exceptionally rare. It becomes then possible to suppose that for a similar
period stimuli to change in the manifestation of animal forms may have been
exceptionally few and feeble,--that is, if the conditions of the earth's
orbit have been as exceptional as stated. However, even if new species are
actually now being evolved as actively as ever, or if they have been so
quite recently, no conflict thence necessarily arises with the view here
advocated. For it by no means follows that if some examples of new species
have recently been suddenly produced from individuals of antecedent
species, we ought to be able to put our fingers on such cases; as Mr.
Murphy well observes[230] in a passage before quoted, "If a species were to
come suddenly into being in the wild state, as the Ancon sheep did under
domestication, how could we ascertain the fact? If the first of a
newly-born species were found, the fact of its discovery would tell nothing
about its origin. Naturalists would register it as a very rare species,
having been only once met with, but they would have no means of knowing
whether it were the first or the last of its race."

But are there any grounds for thinking that in the genesis of species an
_internal_ force or tendency interferes, co-operates with and controls the
action of external conditions?

It is here contended that there are such grounds, and that though
inheritance, reversion, atavism, Natural Selection, &c., play a part not
unimportant, yet that such an internal power is a great, perhaps the main,
determining agent.

It will, however, be replied that such an entity is no _vera causa_; that
if the conception is accepted, it is no real explanation; and that it is
merely a roundabout way of saying that the facts are as they are, while the
cause remains unknown. To this it may be rejoined that for all who believe
in the existence of the abstraction "force" at all, other than will, {228}
this conception of an internal force must be accepted and located
somewher