e are naturally led
to suppose that the two series have been separately created. When taking
a more limited view we see a river, with a quite similar country on both
sides, with one side well stocked with a certain animal and on the other
side not one (as is the case with the Bizcacha{376} on the opposite
sides of the Plata), we are at once led to conclude that the Bizcacha
was produced on some one point or area on the western side of the
river. Considering our ignorance of the many strange chances of
diffusion by birds (which occasionally wander to immense distances) and
quadrupeds swallowing seeds and ova (as in the case of the flying
water-beetle which disgorged the eggs of a fish), and of whirlwinds
carrying seeds and animals into strong upper currents (as in the case of
volcanic ashes and showers of hay, grain and fish{377}), and of the
possibility of species having survived for short periods at intermediate
spots and afterwards becoming extinct there{378}; and considering our
knowledge of the great changes which _have_ taken place from subsidence
and elevation in the surface of the earth, and of our ignorance of the
greater changes which _may have_ taken place, we ought to be very slow
in admitting the probability of double creations. In the case of plants
on mountain-summits, I think I have shown how almost necessarily they
would, under the past conditions of the northern hemisphere, be as
similar as are the plants on the present Arctic shores; and this ought
to teach us a lesson of caution.

{375} On centres of creation see _Origin_, Ed. i. p. 352, vi. p.
499.

{376} In the _Journal of Researches_, Ed. 1860, p. 124, the
distribution of the Bizcacha is described as limited by the river
Uruguay. The case is not I think given in the _Origin_.

{377} In the _Origin_, Ed. i. a special section (p. 356, vi. p.
504) is devoted to _Means of Dispersal_. The much greater
prominence given to this subject in the _Origin_ is partly
accounted for by the author's experiments being of later date,
_i.e._ 1855 (_Life and Letters_, vol. II. p. 53). The carriage of
fish by whirlwinds is given in the _Origin_, Ed. i. p. 384, vi. p.
536.

{378} The case of islands serving as halting places is given in the
_Origin_, Ed. i. p. 357, vi. p. 505. But here the evidence of this
having occurred is supposed to be lost by the subsidence of the
islands, not mere