nt parts. But besides this, barriers
of every kind seem to separate regions in a greater degree than
proportionally to the difference of climates on each side. Thus great
chains of mountains, spaces of sea between islands and continents, even
great rivers and deserts. In fact the amount <of> difference in the
organisms bears a certain, but not invariable relation to the amount of
physical difficulties to transit{123}.

{122} In the _Origin_, Ed. i. p. 346, vi. p. 493, the author begins
his discussion on geographical distribution by minimising the
effect of physical conditions. He lays great stress on the effect
of _barriers_, as in the present Essay.

{123} Note in the original, "Would it be more striking if we took
animals, take Rhinoceros, and study their habitats?"

There are some curious exceptions, namely, similarity of fauna of
mountains of Europe and N. America and Lapland. Other cases just <the>
reverse, mountains of eastern S. America, Altai <?>, S. India <?>{124}:
mountain summits of islands often eminently peculiar. Fauna generally of
some islands, even when close, very dissimilar, in others very similar.
[I am here led to observe one or more centres of creation{125}.]

{124} Note by Mr A. R. Wallace. "The want of similarity referred
to, is, between the mountains of Brazil and Guiana and those of the
Andes. Also those of the Indian peninsula as compared with the
Himalayas. In both cases there is continuous intervening land.

"The islands referred to were, no doubt, the Galapagos for
dissimilarity from S. America; our own Islands as compared with
Europe, and perhaps Java, for similarity with continental Asia."

{125} The arguments against multiple centres of creation are given
in the _Origin_, Ed. i. p. 352, vi. p. 499.

The simple geologist can explain many of the foregoing cases of
distribution. Subsidence of a continent in which free means of
dispersal, would drive the lowland plants up to the mountains, now
converted into islands, and the semi-alpine plants would take place of
alpine, and alpine be destroyed, if mountains originally were not of
great height. So we may see, during gradual changes{126} of climate on a
continent, the propagation of species would vary and adapt themselves to
small changes causing much extermination{127}. The mountains of Europe
were quite lately covered with ice, and the lowlands probably partakin