No doubt, direct evidence of seeds being carried great distances through the air is wanted, but, I am afraid, can hardly be obtained.  Yet I feel the greatest confidence that they are so carried.  Take for instance the two peculiar orchids of the Azores (Habinaria species):  what other mode of transit is conceivable?  The whole subject is one of great difficulty, but I hope my chapter may call attention to a hitherto neglected factor in the distribution of plants.

Your references to the Mauritius literature are very interesting, and will be useful to me; and again thanking you for your valuable remarks, believe me yours very faithfully,

ALFRED R. WALLACE.

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Pen-y-bryn, St. Peter’s Road, Croydon.  November 21, 1880.

My dear Darwin,—­Many thanks for your new book containing your wonderful series of experiments and observations on the movements of plants.  I have read the introduction and conclusion, which shows me the importance of the research as indicating the common basis of the infinitely varied habits and mode of growth of plants.  The whole subject becomes thus much simplified, though the nature of the basic vitality which leads to such wonderful results remains as mysterious as ever.—­Yours very faithfully,

ALFRED R. WALLACE.

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Pen-y-bryn, St. Peter’s Road, Croydon.  January 1, 1881.

My dear Darwin,—­I have been intending to write to you for some weeks to call your attention to what seems to me a striking confirmation (or at all events a support) of my views of the land migration of plants from mountain to mountain.  In Nature of Dec. 9th, p. 126, Mr. Baker, of Kew, describes a number of the alpine plants of Madagascar as being identical species with some found on the mountains of Abyssinia, the Cameroons, and other African mountains.  Now, if there is one thing more clear than another it is that Madagascar has been separated from Africa since the Miocene (probably the early Miocene) epoch.  These plants must therefore have reached the island either since then, in which case they certainly must have passed through the air for long distances, or at the time of the union.  But the Miocene and Eocene periods were certainly warm, and these alpine plants could hardly have migrated over tropical forest lands, while it is very improbable that if they had been isolated at so remote a period, exposed to such distinct climatal and organic environments as in Madagascar and Abyssinia, they would have in both places retained their specific characters unchanged.  The presumption is, therefore, that they are comparatively recent immigrants, and if so must have passed across the sea from mountain to mountain, for the richness and speciality of the Madagascar forest vegetation render it certain that no recent glacial epoch has seriously affected that island.