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On Barriers

(see top of p. 3)

If we take a general view of distribution, I think we must conclude that barriers, whatever the nature, in regard to powers of passage of organisms is one chiefly, I shd say decidedly the most important element in their distribution. For marine productions, landing stretching N. & S. is a perfect barrier, if it has long existed, so again a wide space of ocean; now compare the shells on each side of I. of Panama, only one the same; so with Crustacea, so with Fish. (Isthmus of Suez so low.) Again there is profound ocean, fully as wide as Atlantic ocean, west of S. America, without an isld. & here there is not a shell in common – but westward is ocean strewed with isld (& with evidence of former isld) the shells & fish extend with very many in common, even to W Coast of Africa, almost exactly as hemisphere. Again land shells of America, correlated with water shells on opposite sides of Alleghenies. (some fish case of Hooker) – so all productions on opposite & alike sides of Cordillera. Looking to land mammals & considering their feeble means of transport, except in certain ice cases & taking soundings as some & the only indications of the chances of finer connection; for no geologist who has studied the astonishing accumulation of evidence of oscillation of level, will think it is the least improbable that land separated by any 200 or 300 fathoms shd have been joined, we shall find very fair evidence

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Barriers

Concordance between identity of species & continuous land. East on E. Indian Archipelago. Celebes case Australia, Van Diemans land, New Guinea — Japan — Madagascar — Asia & Africa & Europe — S. Africa itself deserts lines of sounding in Mediterranean hence also isld of Mediterranean — Grt. Britain. S. America, Falklands soundings, (evidence of ancient subsidence at S. Cruz) but here possibly Ice Action. S. America itself rather a difficulty, formerly whole of America never united with old world, & N & S. America more united & isld of West Indies: Isthmus of Panama does not seem to have been wide enough road. (I am here confounding 2 considerations identity of species & connection of forms, I must reconsider whole case: now I do not know whether any species the same as New Guinea & Australia) (a)

There are a few in common with S. America & N. America. Look at S. Africa, Australia & S. America under same latitude & in parts with very similar climate & how preposterous the idea of any species being the same. The rate on life of forms another consideration to which I have been to recur. – No oceanic isld has same species of mammifer – I do not mean to say that when no barrier the same species occur, for [illeg] other consideration come into play.—

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I think begin with same conditions do not produce same mammalia. — Tropic Africa & Tropical America — sub-tropical Australia & S. Africa… = conditions differ as far as organic being are concerned.=

(a)  Europe some species in common with Africa. give connection oscillation of Gibraltar. depth of Mediterranean — Mediterranean islds Britain  Afri with Asia. Isthmus of Suez & Mediterranean. — Asia & Japan.— Malayan Arch., Celebes East. — Australia & Tasmania — (India with Asia) Asia & Euro Iceland &c. not N. America

Soundings — Glacial action — S. America & N. America. Madagascar.

Climate of course comes into play thus more of S. America common with Europe. Contrast S. America  C. of Good Hope & Australia.

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Barriers

The first great law that there is some higher cause, than climate for geograph Distrib; (as in structure) for America is more distinct

In time of Buffon

Plants. A. Decandolle has remarked that in N. where lands most united (how are sounding in Behring St) & bearing in mind more internal connection anteriorly to glacial period) that most species in common in north & get fewer & fewer in south — We shall have to consider some exceptions far south — Cape of Good Hope one of the most distinct areas in world, deserts & tropical band. Most plants have such small areas that it wd not be fair to compare (& so it is not fair, except in relation to higher connections thus more species of mammals) S. America, S. Africa & Australia. The evidence from plants not good, as we shall see when we come to islds. But yet we see that Barriers have most to do, more species of plants in common between the warmer temperate regions of B. Oriental & S. Brazil, more between tropics of Australia & S. Australia than between S. America & Australia, & C. of Good Hope. In Plants we see well that great distribution without Barriers. East & West Australia, formerly isld preoccupation — Look in S. Africa in A Decandolle I believe very few species common to the 20 areas in which this has been divided.

Refers to whole Barriers are a most important element in distribution, & this I can understand only that species created in one spot, & wanders as far as it can, considering conditions & preoccupation, till it meets Barrier. Perhaps even those who believe in double creation wd admit this & say that double creations are exceptions, & the case perhaps shd be considered, whether former means of passage are not the exceptions.

[in margin:] I think plants had better be discussed earlier

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