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Alexander von Humboldt: „On the Laws observed in the Distribution of vegetable Forms“, in: ders., Sämtliche Schriften digital, herausgegeben von Oliver Lubrich und Thomas Nehrlich, Universität Bern 2021. URL: <https://humboldt.unibe.ch/text/1816-Sur_les_lois-2> [abgerufen am 26.04.2024].
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| Titel | On the Laws observed in the Distribution of vegetable Forms | ||||
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| Jahr | 1816 | ||||
| Ort | London | ||||
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Nachweis in: The Philosophical Magazine and Journal 47:218 (Juni 1816), S. 446–453.
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| Sprache | Englisch | ||||
| Typografischer Befund | Antiqua; Auszeichnung: Kursivierung; Fußnoten mit Asterisken und Kreuzen; Schmuck: Initialen, Kapitälchen. | ||||
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|446|
Botany, long confined to the simple description of the externalforms of plants and their artificial classification, now presentsseveral branches of study, which place it more on a footing withthe other sciences. Such are the distribution of vegetables ac-cording to a natural method founded upon the whole part oftheir structure; physiology, which displays their internal organi-zation; botanical geography, which assigns to each tribe of plantstheir height, limits, and climate. The terms alpine plants,plants of hot countries, plants of the sea-shore, are to be foundin all languages, even in those of the most savage nations on thebanks of the Oronoko. They prove that the attention of menhas been constantly fixed on the distribution of vegetables, andon their connexion with the temperature of the air, the elevationof the soil, and the nature of the ground which they inhabit.It does not require much sagacity to observe, that on the slopeof the high mountains of Armenia, vegetables of a different lati-tude follow each in succession, like the climates, superposed as itwere upon each other. This idea of Tournefort, developed byLinnæus in two interesting dissertations, (Stationes et ColoniæPlantarum,) nevertheless contains the first seeds of botanicalgeography. Menzel, the author of an unpublished Flora of Ja-pan, strongly recommends to travellers researches as to the dis-tribution of species in the different regions of the globe. Hehad even pointed out the result before by the name of the Geo-graphy of Plants. This appellation was again employed, andalmost at the same time, about the year 1783, by the AbbéGiraud Soulavie, and by the celebrated author of the Studies ofNature; a work which, amid a great variety of very inaccurateideas as to the physique of the globe, contains some profoundand ingenious views as to the forms, relations, and habitudes ofvegetables. Abbé Giraud Soulavie was occupied in preferencewith the plants already cultivated: he has distinguished the cli-mates of the olive trees, the vines, and the chesnuts. He givesa vertical section of Mount Mezin, to which he has added the
* Extracted from a paper read to the French Institute, Feb. 5, 1816.|447| barometrical heights, “because,” as he says, “he has a greatcontempt for every result taken from barometrical measurement.”His Geography of the Plants in the South of France was followedby the Tentamen Historiæ geographicæ Vegetabilium of thelearned Professor Strohmayer, published in 1800 at Gottingenin the form of a dissertation; but this Tentamen exhibits ratherthe plan of a future work, and the catalogue of authors to beconsulted, than information respecting the altitudes which spon-taneous plants reach in different climates. The case is the samewith the very philosophical views announced by M. Treviranusin his Essai de Biologie; we therein find general considerations,but no measurements of heights, and no thermometrical indica-tions, which are the solid bases of the geography of plants. Thisstudy has not risen to the rank of a science, until men of sciencehave perfected both the measures of heights by barometricalobservations, and the determination of mean temperatures; or,what is more important for the development of vegetation, thedetermination of the differences between the temperature ofsummer and winter and between that of day and night. Fewbranches of study have in our day made more rapid progress; anda long time has not intervened between the first efforts and thepresent period, when by the united observations of a great num-ber of travellers, we have succeeded in fixing the limits of ve-getables in Lapland, the Pyrenees, the Alps, Caucasus, and theCordilleras of America. The vegetables which cover the vast surface of the globe pre-sent, when we study by natural classes or families, striking dif-ferences in the distribution of their forms: it is to the laws of thisdistribution that I have recently turned my attention. On li-miting them to the countries in which the number of the speciesis exactly known *, and by dividing this number by that of the Glumaceæ †, the leguminous plants, the labiated, and the com-pound, we find numerical relations which form very regularseries. We see certain forms become more common from theequator towards the pole, like the ferns, the glumaceæ, the eri-cineæ, and the rhododendrons. Other forms on the contrary in-crease from the poles towards the equator, and may be consi-dered in our hemisphere as southern forms: such are the ru-biaceæ, the malvaceæ, the euphorbia, the leguminous and thecomposite plants. Finally, others attain their maximum evenin the temperate zone, and diminish also towards the equator
* Lapland, France, England, &c. according to Messrs. Wahlenberg,Buch, Ramond, Decandolle, and Smith.† The Glumaceæ contain the three families of Grumineæ, Cyperaceæ, and Juncaceæ. |448| and the poles. Such are the labiated plants, the amentaceæ,the cruciferæ, and the umbelliferæ. Part of these data long sincestruck botanical travellers, and all those who have looked intoherbals. It was known that the cruciferæ and umbelliferæ dis-appeared almost entirely in the plains of the torrid zone, andthat none of the malvaceæ were found beyond the polar circle.It is the same with the geography of the plants as with meteoro-logy. The results of those sciences are so simple that in all agesgeneral ideas have been formed of them: but it is only after la-borious researches, and after having collected a great number ofaccurate observations, that numerical results were attained, andan acquaintance with the partial modifications undergone by thelaw of the distribution of forms. A table which we have drawnup exhibits this law with respect to sixteen families of plantsdistributed over the equatorial, temperate, and glacial zones. Wethere see with satisfaction mixed with surprise, how in organicnature, the forms present constant relations under the same iso-thermal parallels, i. e. on curves traced by points of the globewhich receive an equal quantity of heat. The grasses form inEngland 1-12th, in France 1-13th, in North America 1-10th,of all the phanerogamous plants. The glumaceæ form in Ger-many 1-7th; in France 1-8th; in North America 1-8th; inNew Holland, according to the researches of Mr. Brown, 1-8th;of the known phanerogamous plants. The composite plants in-crease a little in the northern part of the new continent; for,according to the new Flora of Pursch, there is between the pa-rallels of Georgia and Boston 1-6th; whereas in Germany wefind 1-8th; and in France 1-7th, of the total number of the specieswith visible fructification. In the whole temperate zone, theglumaceæ and the composite plants form together; nearly one-fourth of the phanerogamous plants; the glumaceæ, the com-positæ, the cruciferæ and the leguminosæ, together nearly one-third. It results from these researches that the forms of or-ganized beings are in a mutual dependence, and that the unityof nature is such that the forms are limited, the one after theother, according to constant laws easy of determination. Whenwe know upon any point of the globe the number of species pre-sented by one of the great families of the glumaceæ, the com-posite, the cruciferous, or the leguminous plants, we may estimatewith considerable probability both the total number of the pha-nerogamous plants, and the number of species which composethe other vegetable families. It is thus that, by knowing underthe temperate zone the number of the cyperaceæ or compositeplants, we may guess at that of the gramineous or leguminousplants. |449| The number of vegetable species described by botanists, or ex-isting in European herbals, extends to 44000, of which 6000are agamous. In this number we had already included 3000new phanerogamous species enumerated by M. Bonpland andmyself. France, according to M. Decandolle, possesses 3645phanerogamous plants, of which 460 are glumaceæ, 490 com-posite, and 230 leguminous, &c. In Lapland there are only497 phanerogamous plants; among which are 124 glumaceæ,58 composite, 14 leguminous, 23 amentaceous, &c. See myEssay on the Geography of Plants published in 1806, and ofwhich I am preparing a new edition. In order to account for the differences which exist sometimesbetween the relations exhibited by Germany, North America,and France, we must take into consideration the more or lesstemperate climates of those regions. France extends from 42\( \frac{1}{2} \)°to 51° of latitude, On this extent the mean annual heat is 16° 7′to 11°: the mean heats of the summer months are 24° to 19°.Germany, comprised between 46° and 54° of latitude, pre-sents at its extremities mean annual temperatures of 12° 5′ and8° 5′. The mean heats of the summer months there are 21°and 18°. North America, in its immense extent, presents themost varied climates. Mr. Pursch has made us acquainted with2000 phanerogamous plants which grow between the parallelsof 35° and 44°; consequently under mean annual temperaturesof 16° and 7°. The Flora of North America is a mixture ofseveral Floras. The southern regions give it an abundance ofmalvaceæ and composite plants; the northern regions, colder thanEurope under the same parallel, furnish to this Flora abundanceof rhododendrons, amentaceæ, and coniferæ. The caryophylleæ,the umbelliferæ, and the cruciferæ are in general more rare inNorth America than in the temperate zone of the old conti-nent *. These constant relations observed on the surface of the globe,in the plains from the equator to the pole, are again traced inthe midst of perpetual snows on the summits of mountains. Wemay admit, in general, that on the cordilleras of the torrid zonethe boreal forms become more frequent. It is thus that we seeprevail at Quito on the summit of the Andes, the ericineæ, the
* For the sake of those who are not much conversant in descriptive bo-tany, we shall here enumerate the plants which serve as a type to the formsor principal families: Glumaceæ, rushes, tares; orchideæ, orchis, satyrion,vanilla; labiatæ, sage; ericineæ, broom; compositæ, coltsfoot, tussilago; ru-biaceæ, madder, quinquina; umbelliferæ, fennel; cruciferæ, radish, cab-bage; malvaceæ, cotton; leguminosæ, furze, truffles, sensitive plant; euphorbiaceæ, milky thistle; amentaceæ, willow, oak; coniferæ, pine, yew,juniper.|450| rhododendrons, and the gramineous plants. On the contrary,the labiatæ, the rubiaceæ, the malvaceæ, and the euphorbiaceæthen become as rare as they are in Lapland. But this analogyis not supported in the ferns and the composite plants. Thelatter abound on the Andes, whereas the former gradually dis-appear when they rise above 1800 fathoms in height. Thus theclimate of the Andes resembles that of northern Europe onlywith respect to the mean temperature of the year. The repar-tition of heat into the different seasons is entirely different, andpowerfully influences the phænomena of vegetation. In general,the forms which prevail among the Alpine plants are, accordingto my researches, under the torrid zone, the gramineæ (ægopogon, podosæmum, deyeuxia, avena); the compositæ (culcitium, espeletia, aster, baccharis); and the caryophylleæ (arenaria, stellaria). Under the temperate zone, the com-positæ (senecio, leontodon, aster); the caryophylleæ (ceras-tium, cherleria, silene); and the cruciferæ (draba, lepidium). Under the frozen zone, the caryophylleæ (stellaria, alsine);the ericineæ (andromeda) and the ranunculaceæ. These researches into the law of the distribution of forms, na-turally lead to the question whether there exist plants commonto both continents? a question which inspires the more interest,as it belongs to one of the most important problems in Zoonomia.It has been long known, and it is one of the most interestingresults from the geography of animals, that no quadruped, noterrestrial bird, and, as appears from the researches of M. La-treille, almost no insect, is common to the equatorial regions ofthe two worlds. M. Cuvier is convinced by precise inquiriesthat this rule applies even to reptiles. He has ascertained thatthe true boa constrictor is peculiar to America, and that theboas of the old continent were pytons. As to the regions beyondthe tropics, Buffon has multiplied beyond measure the numberof the animals common to America, to Europe, and the north ofAsia. We are assured that the bison, the stag, and the goat ofAmerica, the rabbit and the musk rat, the bear, &c. &c. arespecies entirely different from those of Europe, although Buffonhad affirmed the contrary. There remain only the glutton, thewolf, the white bear, the red fox, perhaps also the elau, whichhave not characters sufficient to entitle them to be specific.Among the plants, we must distinguish between the agamæ andthe cotyledoneæ; and by considering the latter between the mo-nocotylodens and the dicotyledons. There remains no doubtthat many of the mosses and lichens are to be found at once inequinoctial America and in Europe: our herbals show this. Butthe case is not the same with the vascular agamæ as with theagamæ of a cellular texture. The ferus and the lycopodiaceæ |451| do not follow the same laws with the mosses and the lichens.The former, in particular, exhibit very few species universally tobe found; and the examples cited are frequently doubtful. Asto the phanerogamous plants (with the exception of the rhizo-phora, the avicennia, and some other littoral plants), the law ofBuffon seems to be exact with respect to the species furnishedwith two cotyledons. It is absolutely false, although it has beenoften affirmed, that the ridges of the cordilleras of Peru, theclimate of which has some analogy with the climate of Franceor Sweden, produce similar plants. The oaks, the pines, theyews, the ranunculi, the rose-trees, the alchemilla, the valerians,the stellaria, the draba of the Peruvian and Mexican Andes, havenearly the same physiognomy with the species of the same ge-nera of North America, Siberia, or Europe. But all these al-pine plants of the Cordilleras, without excepting one amongthree or four thousand which we have examined, differ specifi-cally from the analogous species of the temperate zone of the oldcontinent. In general, in that part of America situated betweenthe tropies, the monocotyledontal plants alone, and among thelatter almost solely the cyperaceæ and the gramineæ, are com-mon to the two worlds. These two families form an exceptionto the general law which we are here examining,—a law which isso important for the history of the catastrophes of our planet,and according to which the organized beings of the equinoctialregions differ essentially in the two continents. I have given inmy Prolegomena a precise catalogue of those monocotyledontalplants common to the shores of the Oronoko, Germany, and theEast Indies. Their number does not exceed 20 or 24 species,among which it is sufficient to cite the cyperus mucronatus,c. hydra, hypælyptum argenteum, poa eragrostis, andropogon,allioni, &c. In North America placed beyond the tropics, we find nearlyone-seventh of monocotyledontal and dicotyledontal plants com-mon to the two continents. Of 2900 phanerogamous speciescollected in the New Flora of Pursch, 390 are European. It istrue that we may hazard some doubts, as well with respect tothe number of the plants which have accompanied Europeansfrom one hemisphere to the other, as upon those which, whenbetter examined, will be recognised subsequently as new species:but it is impossible that this state of uncertainty should extendto all; and it is to be presumed that, even after a careful exami-nation, the number of the species common to the temperatezones of the two worlds will still remain very considerably ana-logous. Mr. Brown recently undertook some researches on theplants of New Holland. A twenty-eighth part of all the mono-cotyledons hitherto found in the austral continent are common |452| to it with England, France, and Germany. Among the dicoty-ledons the ratio is only 1 in 200; which proves once more how,in the two hemispheres, the grasses and the cyperaceæ are themost diffused, on account of the extreme flexibility of their or-ganization. It would be desirous that learned zoologists shouldendeavour to examine the analogous numerical ratios presentedby the distribution of the different families of animals on theglobe. In the austral hemisphere the vegetable forms of the torridzone advance more towards the pole than in the boreal hemi-sphere. The fern-trees in Asia and America are rarely to befound beyond the tropic of Cancer; whereas in the austral partof our globe the Dicksonia antarctica, the trunk of which is sixmetres in height, pushes its migrations as far as Van Diemen’sLand under the latitude of 42°: it has even been found in NewZealand, in Dusky Gulph, under the parallel of Lyons. Other forms not less majestic, and which were thought to be-long exclusively to the equinoctial Flora, the parasite orchideæ(epidendra, dendrobia) are found mixed with the arborescentferus far beyond the tropic of Capricorn, in the centre of theaustral temperate zone. These phænomena of the geography ofplants prove how vague is what has been generally said of thegreat diminution of temperature in the southern hemisphere,without distinguishing between the parallels more or less nearthe pole, and without any regard to the division of heat amongthe different seasons of the year. Those regions towards whichthe equinoctial forms extend, possess, on account of the immen-sity of the seas which surround them, a true island climate. From the tropic of Capricorn to the parallel of 34°, and per-haps still further, the mean heats of the year (i. e. the quantityof heat received by any given point of the globe) do not differconsiderably in the two hemispheres. On casting our eyes overthe three continents, New Holland, Africa, and America, we findthat the mean annual temperature of Port Jackson (lat. 33° 51′)is 19° 3′ of the centigrade thermometer: that of the Cape ofGood Hope (lat. 33° 55′) 19° 4′; that of the town of BuenosAyres (lat. 34° 36′) 19° 7′. We may be surprised at this greatequality in the distribution of heat by the 34° of austral latitude.Meteorological observations, still more precise, prove that in theboreal hemisphere, under this very parallel of 34°, we find a meantemperature of 19° 8′. On advancing towards the antarctic pole,perhaps even to the parallel of 57°, the temperatures of the twohemispheres differ less in winter than summer. The MalouineIslands, situated in 51° and a half of south latitude, have less in-tense cold in winter than is experienced at London. The meantemperature of Van Diemen’s Land seems to be 10°; it freezes |453| during winter, but not so much as to destroy the fern-trees andthe parasite orchideæ. In the adjoining seas Capt. Cook, in42° of austral latitude, did not see the thermometer fall below+ 6°, 6 in the midst of winter (July). To these very mild win-ters, summers succeed remarkable for an extraordinary coolness.At the southern extremity of New Holland (lat. 42° 41′) thetemperature of the air rarely rises in the midst of summer atnoon-day higher than 12° or 14°; and in Patagonia, as in theadjoining ocean (lat. 48° — 58°), the mean heat of the warmestmonth is only 7° — 8°; whereas in the boreal hemisphere at Pe-tersburg and Umeo (lat. 59° 56′ and 63° 50′) this heat exceeds17 — 19. It is this mild temperature of the islands, whichthe southern countries enjoy between 30° and 40° of latitude,which permits the vegetable forms to pass beyond the tropic ofCapricorn. They embellish a great part of the temperate zone;and the genera which the inhabitant of the northern hemisphereregards as exclusively belonging to the tropical climates, presentnumerous species between the parallels of 35° and 38° of southlatitude.
On the Laws observed in the Distribution of vegetableForms. By Alexander Count Humboldt *.
* Extracted from a paper read to the French Institute, Feb. 5, 1816.|447| barometrical heights, “because,” as he says, “he has a greatcontempt for every result taken from barometrical measurement.”His Geography of the Plants in the South of France was followedby the Tentamen Historiæ geographicæ Vegetabilium of thelearned Professor Strohmayer, published in 1800 at Gottingenin the form of a dissertation; but this Tentamen exhibits ratherthe plan of a future work, and the catalogue of authors to beconsulted, than information respecting the altitudes which spon-taneous plants reach in different climates. The case is the samewith the very philosophical views announced by M. Treviranusin his Essai de Biologie; we therein find general considerations,but no measurements of heights, and no thermometrical indica-tions, which are the solid bases of the geography of plants. Thisstudy has not risen to the rank of a science, until men of sciencehave perfected both the measures of heights by barometricalobservations, and the determination of mean temperatures; or,what is more important for the development of vegetation, thedetermination of the differences between the temperature ofsummer and winter and between that of day and night. Fewbranches of study have in our day made more rapid progress; anda long time has not intervened between the first efforts and thepresent period, when by the united observations of a great num-ber of travellers, we have succeeded in fixing the limits of ve-getables in Lapland, the Pyrenees, the Alps, Caucasus, and theCordilleras of America. The vegetables which cover the vast surface of the globe pre-sent, when we study by natural classes or families, striking dif-ferences in the distribution of their forms: it is to the laws of thisdistribution that I have recently turned my attention. On li-miting them to the countries in which the number of the speciesis exactly known *, and by dividing this number by that of the Glumaceæ †, the leguminous plants, the labiated, and the com-pound, we find numerical relations which form very regularseries. We see certain forms become more common from theequator towards the pole, like the ferns, the glumaceæ, the eri-cineæ, and the rhododendrons. Other forms on the contrary in-crease from the poles towards the equator, and may be consi-dered in our hemisphere as southern forms: such are the ru-biaceæ, the malvaceæ, the euphorbia, the leguminous and thecomposite plants. Finally, others attain their maximum evenin the temperate zone, and diminish also towards the equator
* Lapland, France, England, &c. according to Messrs. Wahlenberg,Buch, Ramond, Decandolle, and Smith.† The Glumaceæ contain the three families of Grumineæ, Cyperaceæ, and Juncaceæ. |448| and the poles. Such are the labiated plants, the amentaceæ,the cruciferæ, and the umbelliferæ. Part of these data long sincestruck botanical travellers, and all those who have looked intoherbals. It was known that the cruciferæ and umbelliferæ dis-appeared almost entirely in the plains of the torrid zone, andthat none of the malvaceæ were found beyond the polar circle.It is the same with the geography of the plants as with meteoro-logy. The results of those sciences are so simple that in all agesgeneral ideas have been formed of them: but it is only after la-borious researches, and after having collected a great number ofaccurate observations, that numerical results were attained, andan acquaintance with the partial modifications undergone by thelaw of the distribution of forms. A table which we have drawnup exhibits this law with respect to sixteen families of plantsdistributed over the equatorial, temperate, and glacial zones. Wethere see with satisfaction mixed with surprise, how in organicnature, the forms present constant relations under the same iso-thermal parallels, i. e. on curves traced by points of the globewhich receive an equal quantity of heat. The grasses form inEngland 1-12th, in France 1-13th, in North America 1-10th,of all the phanerogamous plants. The glumaceæ form in Ger-many 1-7th; in France 1-8th; in North America 1-8th; inNew Holland, according to the researches of Mr. Brown, 1-8th;of the known phanerogamous plants. The composite plants in-crease a little in the northern part of the new continent; for,according to the new Flora of Pursch, there is between the pa-rallels of Georgia and Boston 1-6th; whereas in Germany wefind 1-8th; and in France 1-7th, of the total number of the specieswith visible fructification. In the whole temperate zone, theglumaceæ and the composite plants form together; nearly one-fourth of the phanerogamous plants; the glumaceæ, the com-positæ, the cruciferæ and the leguminosæ, together nearly one-third. It results from these researches that the forms of or-ganized beings are in a mutual dependence, and that the unityof nature is such that the forms are limited, the one after theother, according to constant laws easy of determination. Whenwe know upon any point of the globe the number of species pre-sented by one of the great families of the glumaceæ, the com-posite, the cruciferous, or the leguminous plants, we may estimatewith considerable probability both the total number of the pha-nerogamous plants, and the number of species which composethe other vegetable families. It is thus that, by knowing underthe temperate zone the number of the cyperaceæ or compositeplants, we may guess at that of the gramineous or leguminousplants. |449| The number of vegetable species described by botanists, or ex-isting in European herbals, extends to 44000, of which 6000are agamous. In this number we had already included 3000new phanerogamous species enumerated by M. Bonpland andmyself. France, according to M. Decandolle, possesses 3645phanerogamous plants, of which 460 are glumaceæ, 490 com-posite, and 230 leguminous, &c. In Lapland there are only497 phanerogamous plants; among which are 124 glumaceæ,58 composite, 14 leguminous, 23 amentaceous, &c. See myEssay on the Geography of Plants published in 1806, and ofwhich I am preparing a new edition. In order to account for the differences which exist sometimesbetween the relations exhibited by Germany, North America,and France, we must take into consideration the more or lesstemperate climates of those regions. France extends from 42\( \frac{1}{2} \)°to 51° of latitude, On this extent the mean annual heat is 16° 7′to 11°: the mean heats of the summer months are 24° to 19°.Germany, comprised between 46° and 54° of latitude, pre-sents at its extremities mean annual temperatures of 12° 5′ and8° 5′. The mean heats of the summer months there are 21°and 18°. North America, in its immense extent, presents themost varied climates. Mr. Pursch has made us acquainted with2000 phanerogamous plants which grow between the parallelsof 35° and 44°; consequently under mean annual temperaturesof 16° and 7°. The Flora of North America is a mixture ofseveral Floras. The southern regions give it an abundance ofmalvaceæ and composite plants; the northern regions, colder thanEurope under the same parallel, furnish to this Flora abundanceof rhododendrons, amentaceæ, and coniferæ. The caryophylleæ,the umbelliferæ, and the cruciferæ are in general more rare inNorth America than in the temperate zone of the old conti-nent *. These constant relations observed on the surface of the globe,in the plains from the equator to the pole, are again traced inthe midst of perpetual snows on the summits of mountains. Wemay admit, in general, that on the cordilleras of the torrid zonethe boreal forms become more frequent. It is thus that we seeprevail at Quito on the summit of the Andes, the ericineæ, the
* For the sake of those who are not much conversant in descriptive bo-tany, we shall here enumerate the plants which serve as a type to the formsor principal families: Glumaceæ, rushes, tares; orchideæ, orchis, satyrion,vanilla; labiatæ, sage; ericineæ, broom; compositæ, coltsfoot, tussilago; ru-biaceæ, madder, quinquina; umbelliferæ, fennel; cruciferæ, radish, cab-bage; malvaceæ, cotton; leguminosæ, furze, truffles, sensitive plant; euphorbiaceæ, milky thistle; amentaceæ, willow, oak; coniferæ, pine, yew,juniper.|450| rhododendrons, and the gramineous plants. On the contrary,the labiatæ, the rubiaceæ, the malvaceæ, and the euphorbiaceæthen become as rare as they are in Lapland. But this analogyis not supported in the ferns and the composite plants. Thelatter abound on the Andes, whereas the former gradually dis-appear when they rise above 1800 fathoms in height. Thus theclimate of the Andes resembles that of northern Europe onlywith respect to the mean temperature of the year. The repar-tition of heat into the different seasons is entirely different, andpowerfully influences the phænomena of vegetation. In general,the forms which prevail among the Alpine plants are, accordingto my researches, under the torrid zone, the gramineæ (ægopogon, podosæmum, deyeuxia, avena); the compositæ (culcitium, espeletia, aster, baccharis); and the caryophylleæ (arenaria, stellaria). Under the temperate zone, the com-positæ (senecio, leontodon, aster); the caryophylleæ (ceras-tium, cherleria, silene); and the cruciferæ (draba, lepidium). Under the frozen zone, the caryophylleæ (stellaria, alsine);the ericineæ (andromeda) and the ranunculaceæ. These researches into the law of the distribution of forms, na-turally lead to the question whether there exist plants commonto both continents? a question which inspires the more interest,as it belongs to one of the most important problems in Zoonomia.It has been long known, and it is one of the most interestingresults from the geography of animals, that no quadruped, noterrestrial bird, and, as appears from the researches of M. La-treille, almost no insect, is common to the equatorial regions ofthe two worlds. M. Cuvier is convinced by precise inquiriesthat this rule applies even to reptiles. He has ascertained thatthe true boa constrictor is peculiar to America, and that theboas of the old continent were pytons. As to the regions beyondthe tropics, Buffon has multiplied beyond measure the numberof the animals common to America, to Europe, and the north ofAsia. We are assured that the bison, the stag, and the goat ofAmerica, the rabbit and the musk rat, the bear, &c. &c. arespecies entirely different from those of Europe, although Buffonhad affirmed the contrary. There remain only the glutton, thewolf, the white bear, the red fox, perhaps also the elau, whichhave not characters sufficient to entitle them to be specific.Among the plants, we must distinguish between the agamæ andthe cotyledoneæ; and by considering the latter between the mo-nocotylodens and the dicotyledons. There remains no doubtthat many of the mosses and lichens are to be found at once inequinoctial America and in Europe: our herbals show this. Butthe case is not the same with the vascular agamæ as with theagamæ of a cellular texture. The ferus and the lycopodiaceæ |451| do not follow the same laws with the mosses and the lichens.The former, in particular, exhibit very few species universally tobe found; and the examples cited are frequently doubtful. Asto the phanerogamous plants (with the exception of the rhizo-phora, the avicennia, and some other littoral plants), the law ofBuffon seems to be exact with respect to the species furnishedwith two cotyledons. It is absolutely false, although it has beenoften affirmed, that the ridges of the cordilleras of Peru, theclimate of which has some analogy with the climate of Franceor Sweden, produce similar plants. The oaks, the pines, theyews, the ranunculi, the rose-trees, the alchemilla, the valerians,the stellaria, the draba of the Peruvian and Mexican Andes, havenearly the same physiognomy with the species of the same ge-nera of North America, Siberia, or Europe. But all these al-pine plants of the Cordilleras, without excepting one amongthree or four thousand which we have examined, differ specifi-cally from the analogous species of the temperate zone of the oldcontinent. In general, in that part of America situated betweenthe tropies, the monocotyledontal plants alone, and among thelatter almost solely the cyperaceæ and the gramineæ, are com-mon to the two worlds. These two families form an exceptionto the general law which we are here examining,—a law which isso important for the history of the catastrophes of our planet,and according to which the organized beings of the equinoctialregions differ essentially in the two continents. I have given inmy Prolegomena a precise catalogue of those monocotyledontalplants common to the shores of the Oronoko, Germany, and theEast Indies. Their number does not exceed 20 or 24 species,among which it is sufficient to cite the cyperus mucronatus,c. hydra, hypælyptum argenteum, poa eragrostis, andropogon,allioni, &c. In North America placed beyond the tropics, we find nearlyone-seventh of monocotyledontal and dicotyledontal plants com-mon to the two continents. Of 2900 phanerogamous speciescollected in the New Flora of Pursch, 390 are European. It istrue that we may hazard some doubts, as well with respect tothe number of the plants which have accompanied Europeansfrom one hemisphere to the other, as upon those which, whenbetter examined, will be recognised subsequently as new species:but it is impossible that this state of uncertainty should extendto all; and it is to be presumed that, even after a careful exami-nation, the number of the species common to the temperatezones of the two worlds will still remain very considerably ana-logous. Mr. Brown recently undertook some researches on theplants of New Holland. A twenty-eighth part of all the mono-cotyledons hitherto found in the austral continent are common |452| to it with England, France, and Germany. Among the dicoty-ledons the ratio is only 1 in 200; which proves once more how,in the two hemispheres, the grasses and the cyperaceæ are themost diffused, on account of the extreme flexibility of their or-ganization. It would be desirous that learned zoologists shouldendeavour to examine the analogous numerical ratios presentedby the distribution of the different families of animals on theglobe. In the austral hemisphere the vegetable forms of the torridzone advance more towards the pole than in the boreal hemi-sphere. The fern-trees in Asia and America are rarely to befound beyond the tropic of Cancer; whereas in the austral partof our globe the Dicksonia antarctica, the trunk of which is sixmetres in height, pushes its migrations as far as Van Diemen’sLand under the latitude of 42°: it has even been found in NewZealand, in Dusky Gulph, under the parallel of Lyons. Other forms not less majestic, and which were thought to be-long exclusively to the equinoctial Flora, the parasite orchideæ(epidendra, dendrobia) are found mixed with the arborescentferus far beyond the tropic of Capricorn, in the centre of theaustral temperate zone. These phænomena of the geography ofplants prove how vague is what has been generally said of thegreat diminution of temperature in the southern hemisphere,without distinguishing between the parallels more or less nearthe pole, and without any regard to the division of heat amongthe different seasons of the year. Those regions towards whichthe equinoctial forms extend, possess, on account of the immen-sity of the seas which surround them, a true island climate. From the tropic of Capricorn to the parallel of 34°, and per-haps still further, the mean heats of the year (i. e. the quantityof heat received by any given point of the globe) do not differconsiderably in the two hemispheres. On casting our eyes overthe three continents, New Holland, Africa, and America, we findthat the mean annual temperature of Port Jackson (lat. 33° 51′)is 19° 3′ of the centigrade thermometer: that of the Cape ofGood Hope (lat. 33° 55′) 19° 4′; that of the town of BuenosAyres (lat. 34° 36′) 19° 7′. We may be surprised at this greatequality in the distribution of heat by the 34° of austral latitude.Meteorological observations, still more precise, prove that in theboreal hemisphere, under this very parallel of 34°, we find a meantemperature of 19° 8′. On advancing towards the antarctic pole,perhaps even to the parallel of 57°, the temperatures of the twohemispheres differ less in winter than summer. The MalouineIslands, situated in 51° and a half of south latitude, have less in-tense cold in winter than is experienced at London. The meantemperature of Van Diemen’s Land seems to be 10°; it freezes |453| during winter, but not so much as to destroy the fern-trees andthe parasite orchideæ. In the adjoining seas Capt. Cook, in42° of austral latitude, did not see the thermometer fall below+ 6°, 6 in the midst of winter (July). To these very mild win-ters, summers succeed remarkable for an extraordinary coolness.At the southern extremity of New Holland (lat. 42° 41′) thetemperature of the air rarely rises in the midst of summer atnoon-day higher than 12° or 14°; and in Patagonia, as in theadjoining ocean (lat. 48° — 58°), the mean heat of the warmestmonth is only 7° — 8°; whereas in the boreal hemisphere at Pe-tersburg and Umeo (lat. 59° 56′ and 63° 50′) this heat exceeds17 — 19. It is this mild temperature of the islands, whichthe southern countries enjoy between 30° and 40° of latitude,which permits the vegetable forms to pass beyond the tropic ofCapricorn. They embellish a great part of the temperate zone;and the genera which the inhabitant of the northern hemisphereregards as exclusively belonging to the tropical climates, presentnumerous species between the parallels of 35° and 38° of southlatitude.