We must carefully distinguish between three different questions: How many species of plants are described in printed works? how many have been discovered, i. e. are contained in herbariums, though without being described? how many are probably existing on the globe? Murray’s edition of the Linnean system contains, including cryptogamia, only 10042 species. Willdenow, in his edition of the Species Plantarum, between the years 1797 and 1807, had already described 17457 phænogamous species, (from Monandria to Polygamia diœcia.) If we add 3000 cryptogamous species, we obtain the number which Willdenow mentions, viz. 20000 species. More recent researches have shown how much this estimation of the number of species described and contained in herbariums falls short of the truth. Robert Brown counted above 37000 phænogamous plants. (General Remarks on the Botany of Terra Australis, p. 4.) I afterwards attempted to give the geographical distribution (in different parts of the earth already explored), of 44000 phænogamous and cryptogamous plants. (Humboldt, de distributione geographica Plantarum, p. 23.) Decandolle found, in comparing Persoon’s Enchiridium with his Universal System in 12 several families, that the writings of botanists and European herbariums taken together might be assumed to contain upwards of 56000 species of plants. (Essai élementaire de Géographie botanique, p. 62.) If we consider how many species have since that period been described by travellers,—(my expedition alone furnished 3600 of the 5800 collected species of the equinoctial zone),—and if we remember that in all the botanical gardens taken together there are certainly above 25000 phænogamous plants cultivated, we shall easily perceive how much Decandolle’s number falls short of the truth. Completely unacquainted as we still are with the larger portions of the interior of South America,—(Mato-Grosso, Paraguay, the eastern declivity of the Andes, Santa Cruz de la Sierra, and all the countries between the Orinoco, the Rio Negro, the Amazons, and Puruz),—of Africa, Madagascar, Borneo, and Central and Eastern Asia,—the thought rises involuntarily in the mind that we may not yet know the third, or probably even the fifth part of the plants existing on the earth! Drège has collected 7092 species of phænogamous plants in South Africa alone. (See Meyer’s pflanzen geographische Documente, S. 5 and 12.) He believes that the Flora of that district consists of more than 11000 phænogamous species, while on a surface of equal area (12000 German, or 192000 English square geographical miles) von Koch has described in Germany or Switzerland 3300, and Decandolle in France 3645 species of phænogamous plants. I would also recall that even now new Genera, (some even consisting of tall forest trees), are being discovered in the small West Indian Islands which have been visited by Europeans for three centuries, and in the vicinity of large commercial towns. These considerations, which I propose to develop in further detail at the close of the present annotation, make it probable that the actual number of species exceeds that spoken of in the old myth of the Zend-Avesta, which says that “the Primeval Creating Power called forth from the blood of the sacred bull 120000 different forms of plants!”
If, then, we cannot look for any direct scientific solution of the question of how many forms of the vegetable kingdom,—including leafless Cryptogamia (water Algæ, funguses, and lichens), Characeæ, liver-worts, mosses, Marsilaceæ, Lycopodiaceæ, and ferns,—exist on the dry land and in the ocean in the present state of the organic life of our globe, we may yet attempt an approximate method by which we may find some probable “lowest limits” or numerical minima. Since 1815, I have sought, in arithmetical considerations relating to the geography of plants, to examine first the ratios which the number of species in the different natural families bear to the entire mass of the phænogamous vegetation in countries where the latter is sufficiently well known. Robert Brown, the greatest botanist among our cotemporaries, had previously determined the numerical proportions of the leading divisions of the vegetable kingdom; of Acotyledons (Agamæ, Cryptogamic or cellular plants) to Cotyledons (Phanerogamic or vascular plants), and of Monocotyledonous (Endogenous) to Dicotyledonous (Exogenous) plants. He finds the ratio of Monocotyledons to Dicotyledons in the tropical zone as 1 : 5, and in the cold zones of the parallels of 60° N. and 55° S. latitude, as 1 : 2½. (Robert Brown, General Remarks on the Botany of Terra Australis, in Flinders’ Voyage, vol. ii. p. 338.) The absolute number of species in the three leading divisions of the vegetable kingdom are compared together in that work according to the method there laid down. I was the first to pass from these leading divisions to the divisions of the several families, and to consider the ratio which the number of species of each family bears to the entire mass of phænogamous plants belonging to a zone of the earth’s surface. (Compare my memoir entitled De distributione geographica Plantarum secundum cœli temperiem et altitudinem montium, 1817, p. 24-44; and the farther development of the subject of these numerical relations given by me in the Dictionnaire des Sciences naturelles, T. xviii. 1820, p. 422-436; and in the Annales de Chimie et de Physique, T. xvi. 1821, p. 267-292.)
The numerical relations of the forms of plants, and the laws observed in their geographical distribution, may be considered in two very different ways. If plants are studied in their arrangement according to natural families, without regard to their geographical distribution, it is asked, What are the fundamental forms or types of organisation to which the greatest number of species correspond? Are there on the entire surface of the earth more Glumaceæ than Compositæ? Do these two orders make up between them one-fourth part of the whole number of phænogamous plants? What is the proportion of Monocotyledons to Dicotyledons? These are questions of General Phytology, or of the science which investigates the organisation of plants and their mutual connection, or the present state of the entire vegetable world.
If, on the other hand, the species of plants which have been grouped according to the analogy of their structure are considered, not abstractedly, but according to their climatic relations, or according to their distribution over the surface of the earth, we have questions offering quite another and distinct interest. We then examine what are the families which prevail more in proportion to other Phanerogamæ in the torrid zone than towards the polar circle? Are Compositæ more numerous, either in the same geographical latitudes or on the same isothermal lines, in the New than in the Old Continent? Do the forms which gradually lose their predominance in advancing from the equator towards the poles follow a similar law of decrease in ascending mountains situated in the equatorial regions? Do the proportions of particular families to the whole mass of Phanerogamæ differ in the temperate zones, and on equal isothermal lines, north and south of the equator? These questions belong properly to the Geography of Plants, and connect themselves with the most important problems of meteorology and terrestrial physics. The character of a landscape or country is also in a high degree dependent on the predominance of particular families of plants, which render it either desolate or adorned, smiling or majestic. Grasses forming extensive savannahs, Palms and other trees affording food, or social Coniferæ forming forests, have powerfully influenced nations in respect to their material condition, to their manners, to their mental dispositions, and to the more or less rapid development of their prosperity.
In studying the geographical distribution of forms, we may consider species, genera, and natural families, separately. In social plants, a single species often covers extensive tracts of country; as in northern regions forests of Pines or Firs and extensive heaths (ericeta), in Spain cistus-covered grounds, and in tropical America assemblages of the same species of Cactus, Croton, Brathys, or Bambusa Guadua. It is interesting to examine these relations more closely, and to view in one case the great multiplicity of individuals, and in another the variety of organic development. We may inquire what species produces the greatest number of individuals in a particular zone, or we may ask which are the families to which, in different climates, the greatest number of species belong. In a high northern region, where the Compositæ and the Ferns are to the sum of all the phænogamous plants in the ratio of 1 : 13 and 1 : 25 (i. e. where these ratios are found by dividing the sum total of all the Phanerogamæ by the number of species belonging to the family of Compositæ or to that of Filices or Ferns), it may nevertheless happen that a single species of fern covers ten times more ground than do all the species of Compositæ taken together. In this case Ferns predominate over Compositæ by their mass, or by the number of individuals belonging to the same species of Pteris or Polypodium; but they do not so predominate if we only compare the number of the different specific forms of Filices and Compositæ with the sum of all the phænogamous plants. Since, then, multiplication of plants does not follow the same law in all species,—that is to say, all species do not produce the same number of individuals,—therefore the quotients given by dividing the sum of the phænogamous plants by the number of species belonging to one family, do not suffice by themselves to determine the character of the landscape, or the physiognomy which Nature assumes in different regions of the earth. If the attention of the travelling botanist is engaged by the frequent repetition of the same species, their mass, and the uniformity of vegetation thus produced, it is even more arrested by the rarity or infrequency of several other species which are valuable to mankind. In tropical regions, where the Rubiaceæ, Myrtaceæ, Leguminosæ, or Terebinthaceæ, form forests, one is astonished to find the trees of Cinchona, particular species of Swietenia (Mahogany), Hæmatoxylon, Styrax, and balsamic Myroxylum, so sparingly distributed. We had occasion, on the declivities of the high plains of Bogota and Popayan, and in the country round Loxa, in descending towards the unhealthy valley of the Catamayo and to the Amazons River, to remark the manner in which the trees which furnish the precious fever-bark (species of Cinchona) are found singly and at considerable distances from each other. The China Hunters, Cazadores de Cascarilla (the name given at Loxa to the Indians and Mestizoes who collect each year the most efficacious of all fever-barks, that of the Cinchona Condaminea, among the lonely mountains of Caxanuma, Uritusinga, and Rumisitana), climb, not without peril, to the summits of the loftiest forest trees in order to gain a wide prospect, and to discern the solitarily scattered slender aspiring trunks of the trees of which they are in search, and which they recognise by the shining reddish tint of their large leaves. The mean temperature of this important forest region, situated in 4° to 4½° S. lat. and at an elevation of about 6400 to 8000 English feet, is from 12½° to 16° Réaumur (60°·2 to 68° Fahr.) (Humboldt and Bonpland, Plantes équinoxiales, T. i. p. 88, tab. 10.)
In considering the distribution of species, we may also proceed, without regard to the multiplication of individuals, to the masses which they form or the space which they occupy, and may simply compare together the absolute number of species belonging to a particular family in each country. This is the mode of comparison which Decandolle has employed in the work entitled Regni vegetabilis Systema naturale (T. i. p. 128, 396, 439, 464, and 510), and Kunth has carried it out in regard to the whole number of species of Compositæ at present known (above 3300). It does not show which is the predominant family either in the number of species or in the quantity of individuals as compared with other families; it merely tells how many of the species of one and the same family are indigenous in each country or each quarter of the world. The results of this method are on the whole more exact, because they are obtained by the careful study of single families without the necessity of being acquainted with the whole number of the phanerogamæ belonging to each country. The most varied forms of Ferns, for example, are found between the tropics; it is there, in the tempered heat of moist and shaded places in mountainous islands, that each genus presents the largest number of species: this variety of species in each genus diminishes in passing from the tropical to the temperate zone, and decreases still farther in approaching nearer to the pole. Nevertheless, as in the cold zone—in Lapland, for example—those plants succeed best which can best resist the cold, so the species of Ferns, although the absolute number is less than in France or Germany, are yet relatively more numerous than in those countries; i. e. their number bears a greater proportion to the sum total of all the phanerogamous plants of the country. These proportions or ratios, given as above-mentioned by quotients, are in France and Germany 1⁄73 and 1⁄71, and in Lapland 1⁄25. I published numerical ratios of this kind,—(i. e. the entire quantity of phænogamous plants in each of the different Floras divided by the number of species in each family)—in my Prolegomenis de distributione geographica Plantarum, in 1817; and in the Memoir on the distribution of plants over the Earth’s surface, subsequently published in the French language, I corrected my previously published numbers by Robert Brown’s great works. In advancing from the Equator to the Poles, the ratios taken in this manner vary considerably from the numbers which would be obtained from a comparison of the absolute number of species belonging to each family. We often find the value of the fraction increase by the decrease of the denominator, while yet the absolute number of species has diminished. In the method by fractions, which I have followed as more instructive in reference to the geography of plants, there are two variables; for in proceeding from one isothermal line, or one zone of equal temperature, to another, we do not see the sum total of all the phanerogamæ change in the same proportion as does the number of species belonging to a particular family.
We may, if we please, pass from the consideration of species to that of divisions formed in the natural system of botany according to an ideal series of abstractions, and direct our attention to Genera, to Families, and even to the still higher, i. e. more comprehensive, Classes. There are some genera, and even some entire families, which belong exclusively to particular zones of the Earth’s surface; and this not only because they can only flourish under a particular combination of climatic conditions, but also because both the localities in which they originated, and their migrations, have been limited. It is otherwise with the greater number of genera and of families, which have their representatives in all regions of the globe, and at all latitudes of elevation. The earliest investigations into the distribution of vegetable forms related solely to genera; we find them in a valuable work of Treviranus, in his Biology (Bd. ii. S. 47, 63, 83, and 129). This method is, however, less fitted to afford general results than that which compares either the number of species of each family, or the great leading divisions (of Acotyledons, Monocotyledons, and Dicotyledons) with the sum of all the phanerogamæ. We find that in the cold zones the variety of forms does not decrease so much if estimated by genera as if estimated by species; in other words, we find relatively more genera and fewer species. (Decandolle, Théorie élémentaire de la Botanique, p. 190; Humboldt, Nova genera et species Plantarum, T. i pp. xvi. and 1.) It is almost the same in the case of high mountains whose summits support single members of a large number of genera, which we should have been à priori inclined to regard as belonging exclusively to the vegetation of the plains.
I have thought it desirable to indicate the different points of view from which the laws of the geographical distribution of plants may be considered. It is by confounding these different points of view that apparent contradictions are found; which are unjustly attributed to uncertainties of observation. (Jahrbücher der Gewächskunde, Bd. i Berlin, 1818, S. 18, 21, 30.) When such expressions as the following are made use of—“This form, or this family, diminishes as the cold zones are approached;—it has its true home in such or such a latitude;—it is a southern form;—it predominates in the temperate zone;” care should always be taken to state expressly whether the writer is speaking of the absolute number of species, and its increase or decrease with the change of latitude; or whether he means that the family in question prevails over other families of plants as compared with the entire number of phanerogamæ of which a Flora consists. The impression of prevalence as conveyed by the eye depends on relative quantity.
Terrestrial physics have their numerical elements, as has the System of the Universe, or Celestial Physics, and by the united labours of botanical travellers we may expect to arrive gradually at a true knowledge of the laws which determine the geographical and climatic distribution of vegetable forms. I have already remarked that in the temperate zone the Compositæ (Synanthereæ), and the Glumaceæ (including under this latter name the three families of Grasses, Cyperoidæ and Juncaceæ), make up the fourth part of all phænogamous plants. The following numerical ratios are the results of my investigations for 7 great families of the vegetable kingdom in the same temperate zone.
Glumaceæ 1⁄8 (Grasses alone 1⁄12)
Compositæ 1⁄8
Leguminosæ 1⁄18
Labiatæ 1⁄24
Umbelliferæ 1⁄40
Amentaceæ (Cupuliferæ, Betulineæ, and Salicineæ) 1⁄45
Cruciferæ 1⁄19