From J. D. Hooker [23 February – 6 March 1844]
Acot. & Moncot are done,1 as also a few Dicot.— I expect to come to a rule about several groups of Islds. lying to the W. of large continents of Land in the S. Hemisphere; & hope to prove that they contain a vegetation analogous to that of those continents 20 degrees nearer the adjacent Pole. The Flora of the Galapagos is most allied to that of the S. United States & to that of S. Brazil partially,.— That of St Helena to the Cape—Remotely it is true, but to none other:—of Tristan d’Acunha to what one would suppose the Cape to be if produced to 50 S., where the Antarctic forms would appear— Though the Galapogean Flora is essentially S. American, the proportions of the Nat: Ords to one another is remarkably different, as is the vast quantity of Arborescent Compos. & particularly of Euphorbiaceæ (at which I am now working) & which order has no less than 19 representatives, 16 of them entirely new, they are however of very common genera, Euphorbia, Acalypha Croton & Phyllanthus. Now it is very remarkable, that in an order so poorly represented as Graminæa (& where genera are so mundane), there should be a new genus, & no less than 6 old ones,.—7 genera for 11 species; while in Euphorbiaceæ there are only 4 genera for 19 species—I think in the paucity of grasses there will be a strong analogy to other Tropical Islds;—2 the reverse holds good in the Islands of higher Latitudes. I wish that the Admiralty had returned me my notes that I could draw out some proportions to send you—
With regard to the dissimilarity between the Flora of the several Islds of the group, that is too extraordinary a circumstance for me to offer any remarks upon, until the florula is drawn up, the further I proceed the more I wonder. Had the collections been all made by yourself I should have attributed it to accident, but Macræ’s collections3 are large from Albemarle Isld, & the only 3 plants I have hitherto examined from (Malden Isld (which is it?) are different from those of the other Islets— I was not aware of the analogous fact with regard to the Sandwich group; nor have I yet examined the Canary Isld. Campbell Isld, 2 degrees further S. than Auckland, contains several species not found in the former, though the latter is the smallest & furthest South— I should not think however that it would hold with Islds in the more temperate zones generally, as the Azores which have not very many peculiar plants I shall however sift this subject with my friend Mr H. Watson..4 I have hitherto come to no plants with seeds particularly adapted for transmission; those that afford such facility most markedly, are the Compositæ, & those of the Galaps are the most widely dissimilar from those of any other country of all the Nat. Ords (as far I have seen)— It is I think high time throw overboard laying much stress on the subject of the migration of seeds, except in the cases of lands we know to have been recently formed, or, from devastating causes, to be recently clothed with vegetation— From what I have seen of the collection, I have no reason to suppose that more than one or two of the plants are introduced, even if they were. This leads me to another Question which I am obliged to you for directing my attention to.— “Whether the species of large mundane genera have as wide a range as those of small genera?”— Now the how to set about the solution of this question has puzzled me sorely. In the first place it is not fair to compare the large genus of one Nat. Ord. with its equivalent of small genera of another; & so, a Nat. Ord. must be taken which has a large genus & as many species as that genus contains scattered through a considerable number of smaller ones— Again the Nat: Ord. must be a Mundane one, or there will not be geogr. scope for the observations. This is not all;—Ericeæ is a large mundane Order, but its largest genus Erica is confined to Cape & Europe, & there are none of its species common to both, or any at all in all the New World. Lastly the Nat Ord must be worked out pretty well. There are 3 orders which answer these conditions well—Ranunculaceæ, Cruciferæ & Caryophylleae, then comes another difficulty. There are degrees of distribution. However I put the degrees aside & start with the question in its simplest & boldest form—thus, What species are common to the old & new World?—here two more bugbears occur, at the North where the world gets very small the countries become in a measure identical, & which am I to call Greenland whose Botany is Europæan? that is easily got over by asserting—lastly a Europæan plant being found in N W. America must not be included as a widely spread one, as the Botany of that corner is Identical with Siberia & Kansckatha, & many plants are found only in these two spots.. The following is then my proposition (theorem rather)— What proportion of the Ranunculaceæ of the Old World including Greenland & N W Am (W of the Rocky Mts) is common to the American continent?—& secondly Do the larger proportion of these plants belong to large genera or small.?— In answer to the latter In Ranunculus proper the common are to the whole as1: 10.6. In as many species as R. contains together forming 10 small gen it is. 1: 56.3- Caryophylleæ—In Arenaria 140 species the proportion is1: 28.0 In 15 allied Genera together containing 140 sp. 1: 46.6 In Cerastium a large genus of Caryoph. 69 sp 1: 34.4 In Stellaria of 69 species (a N. Temperate genus) 1: 23.0 but In Silene which is very N. Temperate & has 217 sp.1: 108.8 Cruciferæ— In Arabis & Cardamine together, the two largest genera, 123 sp1: 20.5 of 123 species scattered through about 30 allied genera1: 24.6 In Draba a genus of 77 species (very Arctic however)1: 5.5 These results are any thing but satisfactory & yet the instances are the most strictly comparable I can adduce. In another form, In 855 species comprising 8 genera the proportion of sp. common is to whole 1: 18.2- In 423——————551: 38.5.—
These results may I think be relied on as far as they go, but they would not have been attainable had we not the N. American flora of Torrey & Gray,5 men of unerring sagacity & discrimination— The results, if De Candolle’s6 work alone had been taken, would be erroneous; because he makes species of N. Am plants since discovered to be forms of Europæan, & because the species of the genus have increased in a greater ration than small genera have, & plants common to the two have not turned up in the same proportion— Again De C had not the means of knowing whether some of those common were introduced into the New World or not—Torrey & Gray carefully discriminate these—
What a remarkable fact you mention that the Geog. distrib. of shells is proportional to their persistence in nature. This is a wrinkle to Botanists towards the detection of the orders of fossil plants.. But Cycadeæ are certainly not widely distributed. Pines are (Coniferæ I mean).—7 Do you know any thing of a Mr (Count) Streletski who I hear is in Town & of whom we saw a good deal in V. D. Land?.. 8
I have a list of the principle peat earth plants with an attempt at arranging them according to the proportion each yields: I include more plants than you mention, but your Journal is not before me as I lent it a few days ago— Enclosed is a list of as far as I have gone with the Galapago Isld plants, whenever you return it I will add to it & send it again—I think I have about done, the proportion of new sp. is terrible among Dicot. & I must perpetrate one or two genera.
It hardly appears, either that the genera are distributed equally through all the Islds ,—or that seperate Islds have seperate genera; untill however I have gone through the collection I shall forbear any more remarks, as I am often woefully out when applying the numerical test to my preconceived Ideas— I hope to be at the Geological Soc. this next weeks meeting as I have not seen Mr Lyell or Dr Fitton yet,9 & my Father will go if the weather is tolerable.— I think this letter will tire you out, I wish I could be as useful to you as you are to me in suggesting these most interesting questions from which when properly worked out we may begin to ascend to grand causes
Believe me to remain yours most truly | Jos D Hooker.
Island floras; relationships with mainland. Ranges of species in mundane genera.
Galapagos plants one-third done.
Please cite as
Darwin Correspondence Project, “Letter no. 737,” accessed on 21 October 2016, http://www.darwinproject.ac.uk/DCP-LETT-737