birds are, or have been, distributed around the
antarctic region: as the ostrich in Africa, the rhea in South America, the
emeu in Australia, the apteryx, dinornis, &c. in New Zealand, the epiornis
in Madagascar. Still the existence of such a land would not alone explain
the various geographical cross relations which have been given above. It
would not, for example, account for the resemblance between the crustacea
or fishes of New Zealand and of England. It would, however, go far to
explain the identity (specific or generic) between fresh water and other
forms now simultaneously existing in Australia and South America, or in
either or both of these, and New Zealand.

Again, mutations of elevation small and gradual (but frequent and
intermitting), through enormous periods of time--waves, as it were, of land
rolling many times in many directions--might be made to explain many
difficulties as to geographical distribution, and any cases that remained
would probably be capable of explanation, as being isolated but allied
animal forms, now separated indeed, but being merely remnants of extensive
groups which, at an earlier period, were spread over the surface of the
earth. Thus none of the facts here given are any serious difficulty to the
doctrine of "evolution," but it is contended in this book that if other
considerations render it improbable that the manifestation of the
successive forms of life has been brought about by minute, indefinite, and
fortuitous variations, then these facts as to geographical distribution
intensify that improbability, and are so far worthy of attention.

All geographical difficulties of the kind would be evaded if we could
concede the probability of the independent origin, in different localities,
of the same organic forms in animals high in the scale of nature. {152}
Similar causes must produce similar results, and new reasons have been
lately adduced for believing, as regards the _lowest organisms_, that the
same forms can arise and manifest themselves independently. The difficulty
as to higher animals is, however, much greater, as (on the theory of
evolution) one acting force must always be the ancestral history in each
case, and this force must always tend to go on acting in the same groove
and direction in the future as it has in the past. So that it is difficult
to conceive that individuals, the ancestral history of which is very
different, can be acted upon by all influences, externa