To J. D. Hooker [8 September 1844]1
Down Bromley Kent
My dear Hooker
I ought to have written sooner to have acknowledged your notes & the parcel, of such inestimable value, no doubt, in the eyes of Ehrenberg.— I sent it off the next day, with a few specimens of my own, but few, I fear of any probable interest. Really your collection must have been invaluable for Ehrenberg, now that he is summing up his labours in the distribution of Infusoria.— I have sent Eh. some more specimens of the Atlantic dust; I find that on one occasion, the dust when it began to fall on a ship, more than 300 miles from the land, was much coarser than on the succeeding days & many of the particles of stone are more than 11000 of inch square: this shows how far the sporules of Cryptogams might be blown, & indeed common seeds, by winds of no great force.—
Have you seen Forbes Report;2 it wd interest you in your speculation on ranging: he of all men, in the world, if he can spare time, ought to have the deep sea dredgings.
The subject of the greater number of species in certain areas, than in others, has long appeared to me a very curious subject: your example of East S. America, compared with Britain is very striking. Is not the case of New Zealand, with its varied stations, compared with the uniformly arid C. of Good Hope, opposed to your view, that the number of species bears a relation to the vicissitudes of climate?3 When you speak of mountains, (as the plains of the Andes) being subject to vicissitudes, I am not sure, whether you means absolutely so on the same spot, or whether great differences, within short distances.— Is it not said, that the absolute changes of temperature are greatest on any one spot, in the extreme northern regions; & that equability is the characteristic of the tropics?
The conclusion, which I have come at is, that those areas, in which species are most numerous, have oftenest been divided & isolated from other areas,, united & again divided;—a process implying antiquity & some changes in the external conditions. This will justly sound very hypothetical.
I cannot give my reasons in detail: but the most general conclusion, which the geographical distribution of all organic beings, appears to me to indicate, is that isolation is the chief concomitant or cause of the appearance of new forms (I well know there are some staring exceptions).—4
Secondly from seeing how often plants & animals swarm in a country, when introduced into it, & from seeing what a vast number of plants will live, for instance in England, if kept free from weeds & native plants, I have been led to consider that the spreading & number of the organic beings of any country depend less on its external features, than on the number of forms, which have been there originally created or produced.— I much doubt whether you will find it possible to explain the number of forms by proportional differences of exposure; & I cannot doubt if half the species in any country were destroyed or had not been created, yet that country wd: appear to us fully peopled. With respect to original creation or production of new forms, I have said, that isolation appears the chief element: Hence, with respect to terrestrial productions, a tract of country, which had oftenest within the later geological periods subsided & been converted into islds, & reunited, I shd expect to contain most forms.—
But such speculations are amusing only to one self, & in this case useless as they do not show any direct line of observation: if I had foreseen how hypothetical, the little, which I have unclearly written, I wd not have troubled you with the reading of it.
Believe me,—at last not hypothetically— | Yours very sincerely | C. Darwin
All your remarks are to me of real interest & value.
It used to strike me, that the great apparent change in the vegetation about Chiloe was fully as striking, as the apparent (to my non-botanical eyes) uniformity southward of Chonos Arch: There is no great or sudden change in climate till we reach near Concepcion, where less rain is the chief change.—
Acknowledges note and parcel for Ehrenberg.
Considers why different areas have different numbers of species. Gives an example opposing JDH’s view that paucity of species results from vicissitudes of climate. CD has concluded that species are most numerous in areas that have most often been divided, isolated from, and then reunited with, other areas. Cannot give detailed reasons but believes that "isolation is the chief concomitant or cause of the appearance of new forms".
Please cite as
Darwin Correspondence Project, “Letter no. 776,” accessed on 31 August 2016, http://www.darwinproject.ac.uk/DCP-LETT-776