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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.

    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 (?)¹: 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. . . .

¹. 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."