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Letter 784

Hooker, J. D. to Darwin, C. R.

28 Oct 1844
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    Discusses the connection between climate and vegetation. Believes that an equable climate is unfavourable to increase of species either by importation or modification of existing forms; illustrates his view with reference to particular floras. Hopes to acquire facts to support CD's idea that isolation is important in producing new forms. Considers the floras of islands some of which do have distinctive species but others of which do not. Agrees that the wide ranges of cryptogams are a consequence of their means of dispersal. Asks for references to works on original creation and species mutability in order to get the best notions of "the (mad) theories of some men from Lamarck's twaddle upwards".

Transcription

West Park Kew

October 28. 1844.

My dear Darwin

It is a shameful time since I received your last valued letter, considering it is not answered yet—at the time I received it I had a good deal to say about the ranges of species, but have been so taken up with that dullest of all branches, “specific Botany”, that I fear my ideas “tales quales” have flown or been absorbed.

Your communication was indeed most interesting & much food for me. I think I argued that there was a good deal of concomitancy in a uniformity of temperature & of vegetation, but did not mean you to infer that they were cause & effect: I suppose I have stated the thing too strongly

Still less do I suppose that an equable climate can account for an originally meagre vegetation: but I do think that it both favors the range of an existing one & excludes an increase from without, because many plants accustomed to change are impatient of so constant a drain on their constitutions, & because an equable temperature, causing a vigorous vegetation, tends to cover the surface with a growth of a few large things, which monopolize the soil, both its space & its nutritious qualities—I express myself very ill.

I did not suppose New Zealand with its limited flora opposed to my notion, (pray do not dignify it with the word hypothesis!) because I considered its climate as very equable—Grapes (I was assured) never ripened properly there in 35 & vegetation is luxuriant as far as 52 in the same longitude; & I think its perennially verdant forests, arguing certainly a priori, tend to prove the climate tolerably equable. I should have thought that neither the diurnal or solstitial changes of N. Z. at all equalled those of N. Holl or V. D. L.— It is also a blowy place & the breezes being all sea should be moist— Nor did I forget the Cape, with its sandy plains & abundant variety of vegetation, but had heard from Sir J. Herschell that the surface was heated to a surprizing degree during the day & I thought that nocturnal radiation was very considerable. My friend Mr Harvey tells me that they always require fires in winter, especially towards evening & in the morning. The barren plains of Patagonia & the Plate, district are my greater stumbling blocks, though the Geological formation has much to do with them. Again in judging of New Zealand we do so only of its lee shore, which is a mere modification (not a different one I think) of what prevails on its west which is horrible.

In talking of Mts, I meant certainly absolute changes on the same spot, & grant that these are greatest in the xtreme North, but hardly thought it fair to cite a climate, where the duration & intensity of cold was beyond what all but a very small amount of plants could bear. To exist at all, Phænogamic plants must have a certain amount of heat during some part of the year & shelter during the remainder, or the very latent caloric of their reservoir of life will be absorbed. I think therefore we must not compare a region which only affords 3 months in which it is possible for vegetation to progress, with the Tropics where 12 months allow in some places 12 as many vegetations. But compare the mean temp. of the variable, with that of a similar temperature where it is equable & I think if the Arctic & Antarctic regions be the regions we thus collated, the former will contain most forms for its amount of temperature. Again, I doubt extremely if any 2312 degrees in Lat of Tropics, will contain as many species of plants, as the 2312 to the Northward of it, in the old world at any rate. Except where vast table lands intersecting the tropic give the plants the benefit of a new climate, too changeable for the rank luxuriance of a tropical forest. I do not think any trees have so wide a range as many of the tropical ones. Nothing struck me so much as the few species of plants I was able to gather on the Corcovado at Rio.

Certainly there is not much land, after all, between the Tropics, especially to what there is to the Northward of them; to the Southward there is less still, but in leaving the South. Equinoctial line a new & varied flora is at once met with. The Botany of extra tropical N. H. is far more varied & peculiar than intratropical ditto. There is a vast similarity in the whole Flora of India, from the Chinese sea on the E. to the Zanzibar coast! to the W. & I am led to believe the Guinean flora may be added to it without much violence: but how totally different from any or all is the Flora of S. Africa. I am reasoning (if it be reason) on the broad principle of the tropical being an equable climate.

So much for my side.

I am not prepared to answer your remark concerning the sudden change of vegetation to N. of Chonos. Archip. without concomitant physical features. Allow me however to add, that the main feature of the vegetation to the South of that I suppose to be the Beech— Now the mean temp. of the year I suppose increases along that coast inversely with the Latitude (of course) & there is a temp. that trees cannot bear— Wherever that temp. is arrived at, the Beech will stop, perhaps suddenly, for though plants will “drag on a miserable existence” into climates too cold for their nature, I am not sure that they do so into warm ones. It is worth enquiring whether in the N. Hemisphere the Northern or Southern limits of any tree is most clearly defined?, I think the Southern. You dwell more on the facility of introduction of species into our or other climates, than I used to. After all, considering the hundreds of years our Island has been under cultivation, the 20,000 plants that have at one time or another been introduced into it, the times out of number that the same things have been imported with our foreign produce & which are immediately put into the most favorable situations for being naturalized. After all this, how many plants have we naturalized? Look at any garden neglected for 20 years & how few of the common continental hardy annuals or perennials survive—our equable climate (perhaps) leaves them no room, because it favors the undue increase of our own weeds. Fourteen points of the compass bring land-winds to us & yet how small our flora is compared to the continental one. There are few seasons of the year in which we cannot find some remains of nearly all our native plants: but abroad one vegetation replaces another.

All this however is a paltry subject in comparison to the question you propound. What I have aimed at is, to trace the connection between climate & the present state of vegetation; to account for the paucity of species remaining in an area from a supposition that certain states of climate are unfavorable to increase of species, either by importation or by modification of already existing forms: (if so be that many so called species are permanent alterations, due to climate or other physical cause). I fear no superstructure of inductive reasoning, built upon so narrow a base, even if a stable one, would lead to the solution of your question “The cause of the appearance of new forms.”—

My great ignorance of Geology, or indeed of any thing but specific Botany, prevents my perceiving the truth of your hypothesis pray do not twist this con-fession into any other meaning, I do mean simply that not knowing what parts of the world have been most frequently divided & again united I cannot apply the test of proportionate number of species to it.

I was pondering the other day what materials we had after all for coming to a knowledge of the Geog. distrib. of plants, & was forced to confess it very small, from travellers & collectors invariably neglecting all but the most interesting things & from their passing by a thing of the utmost importance in the physical features of one area, because they gathered it in the last district passed through. We now do not know what 1000 square miles on the Earths surface produces most plants. Every Botanist has been crazy about the extraordinary richness of N. Zealand, because in the multitudes of new plants they overlook the great want of old ones—& because no large collector has ever been there to distribute his plants to all Botanists, who would thus find that every succeeding one brought home the same things.

Am I right in supposing from your hypothesis that Islds. produce most new forms: ceteris paribus. There are glaring examples on both sides. Kerg. Land, one of the most isolated spots in the universe has the Flora of Fuegia, with one two exceptions of which the Cabbage is an unanswerable one. Certainly Auck Id. & Camp. Isld. have a very extraordinary number of new plants for their vegetation, but I expect many of them to exist in South New Zealand. The Falklands have hardly a single peculiar plant. Iceland has none, with 400 sp. a most extensive flora for such a region.

However if you have reached this length of my letter you will be weary enough & so I shall drop the subject.

I have written to friends to be particular in the Bot. geog., especially with regard to the richness of areas & I expect very careful observations from N. India, Ceylon, & Brazil. Nothing will give me so much pleasure as to get grounds for your reasonings & to carry out your theory of isolation.

I am quite agreed that Cryptogamic plants may have been & may be disseminated by winds &c, whether they will always grow where they may now be blown is another question: they may have occupied a small area on the original creation, & all or many may still be present over a great portion of the globe at this present time but only take root where they have done before. Most have such marvellous ranges that I think we do no violence to nature in supposing that of them only one individual was originally created, but that very few years sufficed to people the world with them. How phenogamic plants were circumstanced at the same time is a deep problem. One creation of Cryptogs would people the world as it now appears, whatever changes it may have gone through, short of incineration of the whole sphere!—but with animals & plants of other classes the conclusion is not so conformable to our notions.

Pray what writings on the subject of original creation will give me the best notions of the (mad) theories of some men from Lamarck's twaddle upwards. Species (or what we call species) may be muteable but I should not think they set about it themselves so systematically as he says.

And now all the previous is only a prelude to my main object of writing to you (don't laugh) which was to ask if you have seen a paper in the Tasmanian phil. Soc. Journal on “Atmospheric deposits of dust & ashes with remarks on drift pumice of the coasts of N.H. by Rev. W. B. Clarke, MA., FG.S., C.M.Z.S.,”?— if not I will send it you—

With our kind regards to Mrs Darwin | Believe me ever yours | Jos D Hooker.

I have a paper on an Alga (allied to your ones on glass) it is a new genus which makes the red sea red!—according to somebody.— Montagne I see published in French institute Have you seen it?

    Footnotes Add

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    f1 784.f1
    New Zealand runs from c. 34o to 47o S, but Hooker is extending his definition southward to 52o S to include the Aucklands and Campbell Island.
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    f2 784.f2
    New Holland, now Australia, and Van Diemen's Land, now Tasmania.
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    f3 784.f3
    The caloric theory of heat, in which heat is held to be a material substance, was popular until the middle years of the century.
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    f4 784.f4
    Hooker is probably referring to Thomas Thomson in India, Robert Schomburgk in Brazil, and George Gardner in Ceylon, all three associated with Kew Gardens.
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    f5 784.f5
    Clarke 1842.
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    f6 784.f6
    Montagne 1844.
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    f7 784.f7
    Coral reefs, where the frontispiece map is coloured blue for atolls and lagoon islands, and red for fringing reefs. CD believed the blue-coloured areas indicated subsiding land. See also the first of the two CD notes transcribed with the annotations to this letter.
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    f8 784.f8
    Pin holes in the notes and the letter indicate that they were at one stage attached to each other. The notes are bound immediately following the letter, DAR 100: 22–3.
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