There is good evidence that S.E.
England was dry land during the Glacial period.

I forget what Austen says, but Mammals prove, I think, that England has been united to the Continent since the Glacial period.

I don't see your difficulty about what I say on the breaking of an isthmus: if Panama was broken through would not the fauna of the Pacific flow into the W.Indies, or vice versa, and destroy a multitude of creatures?
Of course I'm no judge, but I thought De Candolle had made out his case about small areas of trees.
You will find at page 112, 3rd edition "Origin," a too concise allusion to the Madeira flora being a remnant of the Tertiary European flora.
I shall feel deeply interested by reading your botanical difficulties against occasional immigration.

The facts you give about certain plants, such as the heaths, are certainly very curious.

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