Dear old Darwin

I have been atrociously busy & am so still—but have all along been itching to have a shot at you. I hear you are disgustingly well & entitled to no sort of consideration— How I should chuckle when well, if I were an habitual invalid, & feel I was cheating my life & old time out of what they would have witheld if they could. Your restored health has brought peace to me— long may it last. The Busks come here today for the night & the Lyells meet them at dinner— I hope Lady L. will take sufficiently to Mrs. Busk who is no end of times better than some of the Lyells friends.¹ All the same I cannot add that she is a special favorite of mine.

As to this Agassian affair,² I wish I could explain to you my crude notions as to glacial period & your position towards it & the Universe at large—³

I suppose I hold this doctrine, that there was a glacial period, but that it was not one of universal cold; because I think that the existing distributer of glaciers is sufficiently demonstrative of the proposition, that by comparatively slight redispositions of sea & land & perhaps axis of globe, you may account for all the leading Palæontological phenomena upon which the glacial period is established, & more than all the purely geological & existing Botanical phenomena by which it is supported. Remember this, that the extent of the glacial action is deduced from 3 phenomena, 1. glacial blocks boulders &c. a purely geological argument, that does not carry the action very far south;—2 Palæontology—which carries the action further & wider:—3 existing distribution of existing plants & animals, which carries its action across every latitude & according to you simultaneously in every longitude. ⁴

Now the value of these phenomena differs greatly.— the Geological is irrefragable— the Palæontological is open to the objection, that the glacial organisms may have been suited to warmer distribution then than now—or that under a less struggle for existence they had a wide climatic distribution. The third phenomenon is unsupported by the 1^st & 2^d. & is open to the further objections that the tropical alpine dispersion of glacial plants may be accounted for by the action of birds winds &c, that the phenomena of tropical vegetation, whether as to its extent, variety, or distribution, is against it,⁵ & that the Physicists deny its possibility on grounds as good as those of existing plants.

You attach little importance to the Physicists objection;⁶—I attach great;—because in the main, physical phenomena regulate the production & distribution of organisms; & because the your theory in contrast to it is a crude one not in harmony with the fact, that the relations of Life to conditions are intricate, & the results can only be accounted for after exploring a Labyrinth of conflicting facts. This is why I call yours a sledge-hammer hypothesis. I know no phenomena so subtle or so difficult to attach their true value to, as this of the distribution of Arctic & temperate types over tropical mts.⁷ To account for it at all, requires a far greater amount of Geographical change than you are disposed to admit.—eg in the case of Panama, which must have been occupied by mountains at least 4 or 5000 feet higher than at present; whilst the dispersion of temperate forms from Japan to Tasmania & from Algeria to the Cape without intermediate mts requires what every botanist will consider an extinction of thousands of tropical genera & even orders. You are surely illogical when you found on the distribution of a very few temperate genera & species, a glacial extension that does violence to a host of tropical species genera & orders.⁸ You are illogical when you deny to the physicist the right to maintain as an argument in favor of this absence of a colder period, the phenomena of tropical distribution, & claim that of temperate in your own justification.—

The question resolves itself into this—putting aside the physicists has there been time enough since the glacial epoch, to have repeopled the tropics with its forms, & to have distributed them as they are now distributed.— if yes, then why has so little change been produced in the same time in temperate latitudes. But here you may ask, is the tropical differentiation greater than the temperate & is tropical distribution of types more general than temperate. I think it is, but to answer that one must see how many orders genera are common in each Zone to the several continents. & balance results.

Your argument drawn from the fact, that a geologist had a right to insist on elevation before the means of elevation were shown,⁹ is not applicable in this case. I do not deny your universal cold period because you cannot show how the cold was brought about, but because the Physicists profess to prove that the earth must have been hotter if anything at your Epoch—¹⁰ It is not as if it were an open question whether the earth were then hotter or colder or equal in temperature. if it were so I would go along with you a great way.

With regard to tropical orchids requiring cool treatment,¹¹ that is no argument, they are cool climate species & were killed before by the heat— no tropical orchid is cultivable in a cool house even when it is never exposed to frost.

I want to get to Down soon but have little prospect, just now. except for a Sunday soon¹²

Ever Yr affec | J D Hooker