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.