Chapter IV: Part 4

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 4-1/2° S. lat. and at an elevation of about 6400 to 8000 English feet, is from 12-1/2° 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

The forms of organic beings are in reciprocal dependence on each other. In the unity of nature these forms limit each other according to laws which are probably attached to periods of long duration. If on any particular part of the globe we know with accuracy the number of species of one of the great families of Glumaceæ, Leguminosæ, or Compositæ, we may with a tolerable degree of probability form approximative inferences, both as to the sum of all the phanerogamæ of the country, and also as to the number of species belonging to the rest of the leading families of plants. The number of Cyperoidæ determines that of Compositæ, and the number of Compositæ that of Leguminosæ; they even enable us to judge in what classes or orders the Floras of countries are still incomplete, and teach us, if we are on our guard against confounding together very different systems of vegetation, what harvest may still remain to be reaped in the several families.

The comparison of the numerical ratios of families in different already well explored zones, has conducted me to the recognition of laws according to which, in proceeding from the equator to the poles, the vegetable forms constituting a natural family decrease or increase as compared with the whole mass of phanerogamæ belonging to each zone. We have here to regard not only the direction of the change (whether an increase or a decrease), but also its rapidity or measure. We see the denominator of the fraction which expresses the ratio increase or decrease: let us take as our example the beautiful family of Leguminosæ, which decreases in going from the equinoctial zone towards the North Pole. If we find its proportion or ratio for the torrid zone (from 0° to 10° of latitude) at 1/10, we obtain for the part of the temperate zone which is between 45° and 52° latitude 1/18, and for the frigid zone (lat. 67° to 70°) only 1/35. The direction followed by the great family of Leguminosæ (increase on approaching the equator), is also that of the Rubiaceæ, the Euphorbiaceæ, and especially the Malvaceæ. On the contrary, the Grasses and Juncaceæ (the latter still more than the former), diminish in approaching the equator, as do also the Ericeæ and Amentaceæ. The Compositæ, Labiatæ, Umbelliferæ, and Cruciferæ, decrease in proceeding from the temperate zone, either towards the pole or towards the equator, the Umbelliferæ and Cruciferæ decreasing most rapidly in the last-named direction; while at the same time in the temperate zone the Cruciferæ are three times more numerous in Europe than in the United States of North America. On reaching Greenland the Labiatæ have entirely disappeared with the exception of one, and the Umbelliferæ with the exception of two species; the entire number of phænogamous species, still amounting, according to Hornemann, to 315 species.

It must be remarked at the same time that the development of plants of different families, and the distribution of vegetable forms, does not depend exclusively on geographical, or even on isothermal latitude; the quotients are not always on the same isothermal line in the temperate zone, for example, in the plains of North America and those of the Old Continent. Within the tropics there is a very sensible difference between America, India, and the West Coast of Africa. The distribution of organic beings over the surface of the earth does not depend wholly on thermic or climatic relations, which are of themselves very complicated, but also on geological causes almost unknown to us, belonging to the original state of the earth, and to catastrophes which have not affected all parts of our planet simultaneously. The large pachydermatous animals are at the present time wanting in the New Continent, while we still find them in analogous climates in Asia and Africa. These differences ought not to deter us from endeavouring to search out the concealed laws of nature, but should rather stimulate us to the study of them through all their intricacies.

The numerical laws of the families of plants, the often striking agreement of the numbers expressing their ratios, where yet the species of which the families consist are for the most part different, conduct us into the mysterious obscurity which envelopes all that is connected with the fixing of organic types in the species of plants and animals, or with their original formation or creation. I will take as examples two adjoining countries which have both been thoroughly explored--France and Germany. In France, many species of Grasses, Umbelliferæ and Cruciferæ, Compositæ, Leguminosæ, and Labiatæ, are wanting which are common in Germany; and yet the numerical ratios of these six great families are almost identical in the two countries, as will be seen by the subjoined comparison.

Families. Germany. France.
Gramineæ. 1/13 1/13
Umbelliferæ. 1/22 1/21
Cruciferæ. 1/18 1/19
Compositæ. 1/8 1/7
Leguminosæ. 1/18 1/16
Labiatæ. 1/26 1/24

This agreement in the number of species in each family compared to the whole number of phenogamous species in the Floras of France and Germany, would not by any means exist if the German species which are missing in France were not replaced there by other types belonging to the same families. Those who are fond of imagining gradual transformations of species, and suppose the different kinds of parrots proper to two islands not far removed from each other to present examples of such a change, will be inclined to attribute the remarkable similarity between the two columns of figures which have just been given, to a migration of species, which, having been the same at first, have been altered gradually by the long-continued action of climatic causes during thousands of years, so that their identity being lost they appear to replace each other. But why is it that our common heather (Calluna vulgaris), why is it that our oaks have never advanced to the eastward of the Ural Mountains, and so passed from Europe to Northern Asia? Why is there no species of the genus Rosa in the Southern Hemisphere, and why are there scarcely any Calceolarias in the Northern Hemisphere? The necessary conditions of temperature are insufficient to explain this. Thermic relations alone cannot, any more than the hypothesis of migrations of plants radiating from certain central points, explain the present distribution of fixed organic forms. Thermic relations are hardly sufficient to explain the limits beyond which individual species do not pass, either in latitude towards the pole at the level of the sea, or in vertical elevation towards the summits of mountains. The cycle of vegetation in each species, however different its duration may be, requires, in order to be successfully passed through, a certain minimum of temperature. (Playfair, in the Transactions of the Royal Society of Edinburgh, vol. v. 1805, p. 202; Humboldt, on the sum of the degrees of temperature required for the cycle of vegetation in the Cerealia, in Mem. sur les lignes isothermes, p. 96; Boussingault, Economie rurale, T, ii. p. 659, 663, and 667; Alphonse Decandolle sur les causes qui limitent les espèces végétales, 1847, p. 8.) But all the conditions necessary for the existence of a plant, either as diffused naturally or by cultivation,--conditions of latitude or minimum distance from the pole, and of elevation or maximum height above the level of the sea,--are farther complicated by the difficulty of determining the commencement of the thermic cycle of vegetation, and by the influence which the unequal distribution of the same quantity of heat into groups of successive days and nights exercises on the excitability, the progressive development, and the whole vital process; to all this must be farther added hygrometric influences and those of atmospheric electricity.

My investigations respecting the numerical laws of the distribution of forms may possibly be applied at some future day with advantage to the different classes of Rotiferæ in the animal creation. The rich collections at the Museum d’Histoire Naturelle in the Jardin des Plantes at Paris, already contained, in 1820, (according to approximate estimations) above 56000 phænogamous and cryptogamous plants in herbariums, 44000 insects (a number doubtless too small, though given me by Latreille), 2500 species of fish, 700 reptiles, 4000 birds, and 500 mammalia. Europe has about 80 species of indigenous mammalia, 400 birds, and 30 reptiles. In the Northern temperate zone, therefore, the species of birds are five times more numerous than those of mammalia, as there are in Europe five times as many Compositæ as there are Amentaceæ and Coniferæ, and five times as many Leguminosæ as there are Orchideæ and Euphorbiaceæ. In the southern hemisphere the ratio of mammalia is in tolerably striking agreement, being as 1 to 4·3. Birds, and still more reptiles, increase in the number of species in approaching the torrid zone more than the mammalia. Cuvier’s researches might lead us to believe that the proportion was different in the earlier state of things, and that many more mammalia had perished by revolutions of Nature than birds. Latreille has shewn what groups of insects increase towards the pole, and what towards the equator. Illiger has given the countries of 3800 species of birds according to the quarters of the globe: it would have been much more instructive if the same thing had been done according to zones. We should find little difficulty in comprehending how on a given space of the earth’s surface the individuals of a class of plants or animals limit each other’s numbers, or how, after long continued contest and many fluctuations caused by the requirements of nourishment and mode of life, a state of equilibrium should be at last established; but the causes which have limited not the number of individuals of a form, but the forms themselves, in a particular space, and founded their typical diversity, are placed beneath the impenetrable veil which still conceals from our eyes all that relates to the manner of the first creation and commencement of organic beings.

If, then, we would attempt to solve the question spoken of in the early part of this dissertation, by giving in an approximate manner the numerical limit, (le nombre limite of French mathematicians), which the whole phanerogamæ now existing on the surface of the earth cannot be supposed to fall short of, we may perhaps find our safest guide in a comparison of the numerical ratios (which, as we have seen, may be assumed to exist between the different families of plants), with the number of species contained in herbariums and cultivated in our great botanic gardens. I have said that in 1820 the number of species contained in the herbariums of the Jardin des Plantes at Paris was already estimated at 56000. I do not permit myself to conjecture the amount which the herbariums of England may contain; but the great Paris herbarium, which was formed with much personal sacrifice by Benjamin Delessert, and given by him for free and general use, was stated at his death to contain 86000 species; a number almost equal to that which, as late as 1835, was conjecturally assigned by Lindley as that of all the species existing on the whole earth. (Lindley, Introduction to Botany, 2d edit. p. 504.) Few herbariums have been reckoned with care, after a complete and strict separation and withdrawal of all mere varieties. Not a few plants contained in smaller collections are still wanting in the greater herbariums which are supposed to be general or complete. Dr. Klotzsch estimates the present entire number of phænogamous plants in the great Royal Herbarium at Schöneberg, near Berlin, of which he is the curator, at 74000 species.

Loudon’s useful work, Hortus Britannicus, gives an approximate view of all the species which are, or at no remote time have been, cultivated in British gardens: the edition of 1832 enumerates, including indigenous plants, exactly 26660 phænogamous species. We must not confound with this large number of plants which have grown or been cultivated at any time and in any part of the whole British Islands, the number of living plants which can be shewn at any single moment of time in any single botanic garden. In this last-named respect the Botanic Garden of Berlin has long been regarded as one of the richest in Europe. The fame of its extraordinary riches rested formerly only on uncertain and approximate estimations, and, as my fellow-labourer and friend of many years’ standing, Professor Kunth, has justly remarked (in manuscript notices communicated to the Gartenbau-Verein in December 1846), “no real enumeration or computation could be made until a systematic catalogue, based on a rigorous examination of species, had been prepared. Such an enumeration has given rather above 14060 species: if we deduct from this number 375 cultivated Ferns, we have remaining 13685 phænogamous species; among which we find 1600 Compositæ, 1150 Leguminosæ, 428 Labiatæ, 370 Umbelliferæ, 460 Orchideæ, 60 Palms, and 600 Grasses and Cyperaceæ. If we compare with these numbers those of the species already described in recent works,--Compositæ (Decandolle and Walpers) about 10000; Leguminosæ, 8070; Labiatæ (Bentham), 2190; Umbelliferæ, 1620; Grasses, 3544; and Cyperaceæ (Kunth, Enumeratio Plantarum), 2000;--we shall perceive that the Berlin Botanic Garden cultivates, of the very large families (Compositæ, Leguminosæ, and Grasses), only 1-7th, 1-8th, and 1-9th;--and of the small families (Labiatæ and Umbelliferæ), about 1-5th, or 1-4th, of described species. If, then, we estimate the number of all the different phænogamous plants cultivated at one time in all the botanic gardens of Europe at 20000, we find that the cultivated species appear to be about the eighth part of those which are already either described or preserved in herbariums, and that these must nearly amount to 160000. This estimate need not be thought excessive, since of many of the larger families, (for example, Guttiferæ, Malpighiaceæ, Melastomeæ, Myrtaceæ, and Rubiaceæ), hardly a hundredth part are found in our garden.” If we take the number given by Loudon in his Hortus Britannicus (26660 species) as a basis, we shall find, (according to the justly drawn succession of inferences of Professor Kunth, in the manuscript notices from which I have borrowed the above), the estimate of 160000 species rise to 213000; and even this is still very moderate, for Heynhold’s Nomenclator botanicus hortensis (1846) even rates the phænogamous species then cultivated at 35600; whereas I have employed Loudon’s number for 1832, viz. 26660. On the whole it would appear from what has been said,--and the conclusion is at first sight a sufficiently striking one,--that at present there are almost more known species of phænogamous plants (with which we are acquainted by gardens, descriptions, or herbariums), than there are known insects. According to the average of the statements which I have received from several of our most distinguished entomologists whom I have had the opportunity of consulting, the number of insects at present described, or contained in collections without being described, may be taken at between 150000 and 170000 species. The rich Berlin collection does not contain less than 90000 species, among which are about 82000 Coleoptera. A very large number of plants have been collected in distant parts of the globe, without the insects which live on them or near them being brought at the same time. If, however, we limit the estimates of numbers to a single part of the world, and that the one which has been the best explored in respect to both plants and insects, viz. Europe, we find a very different proportion; for while we can hardly enumerate between seven and eight thousand European phænogamous plants, more than three times that number of European insects are already known. According to the interesting communications of my friend Dohrn at Stettin, 8700 insects have already been collected from the rich Fauna of that vicinity, (and many micro-Lepidopteræ are still wanting), while the phænogamous plants of the same district scarcely exceed 1000. The Insect Fauna of Great Britain is estimated at 11600 species. Such a preponderance of animal forms need the less surprise us, since large classes of insects subsist solely on animal substances, and others on agamous vegetation (funguses, and even those which are subterranean). Bombyx pini alone (the spider which infests the Scotch fir, and is the most destructive of all forest insects), is visited, according to Ratzeburg, by thirty-five parasitical Ichneumonides.

If these considerations have led us to the proportion borne by the species of plants cultivated in gardens to the entire amount of those which are already either described or preserved in herbariums, we have still to consider the proportion borne by the latter to what we conjecture to be the whole number of forms existing upon the earth at the present time; _i. e._ to test the assumed minimum of such forms by the relative numbers of species in the different families, therefore, by uncertain multipliers. Such a test, however, gives for the lowest limit or minimum number results so low as to lead us to perceive that even in the great families,--our knowledge of which has been of late most strikingly enriched by the descriptions of botanists,--we are still acquainted with only a small part of existing plants. The Repertorium of Walpers completes Decandolle’s Prodromus of 1825, up to 1846: we find in it, in the family of Leguminosæ, 8068 species. We may assume the ratio, or relative numerical proportion of this family to all phænogamous plants, to be 1/21--as we find it 1/10 within the tropics, 1/18 in the middle temperate, and 1/33 in the cold northern zone. The _described_ Leguminosæ would thus lead us to assume only 169400 existing phænogamous species on the whole surface of the earth, whereas, as we have shewn, the Compositæ indicate more than 160000 already _known_ species. The discordance is instructive, and may be further elucidated and illustrated by the following analogous considerations.

The major part of the Compositæ, of which Linnæus knew only 785 species and which has now grown to 12000, appear to belong to the Old Continent: at least Decandolle described only 3590 American, whilst the European, Asiatic, and African species amounted to 5093. This apparent richness in Compositæ is, however, illusive, and considerable only in appearance; the ratio or quotient of the family, (1/15 between the tropics, 1/7 in the temperate zone, and 1/13 in the cold zone), shews that even more species of Compositæ than Leguminosæ must hitherto have escaped the researches of travellers; for a multiplication by 12 would give us only the improbably low number of 144000 Phænogamous species. The families of Grasses and Cyperaceæ give still lower results, because comparatively still fewer of their species have been described and collected. We have only to cast our eyes on the map of South America, remembering the wide extent of territory occupied by grassy plains, not only in Venezuela and on the banks of the Apure and the Meta, but also to the south of the forest-covered regions of the Amazons, in Chaco, Eastern Tucuman, and the Pampas of Buenos Ayres and Patagonia, bearing in mind that of all these extensive regions the greater part have never been explored by botanists, and the remainder only imperfectly and incompletely so. Northern and Central Asia offer an almost equal extent of Steppes, but in which, however, dicotyledonous herbaceous plants are more largely mingled with the Gramineæ. If we had sufficient grounds for believing that we are now acquainted with half the phænogamous plants on the globe, and if we took the number of known species only at one or other of the before-mentioned numbers of 160000 or 213000, we should still have to take the number of grasses (the general proportion of which appears to be 1/12), in the first case at least at 26000, and in the second case at 35000 different species, which would give respectively in the two cases only either 1/8 or 1/10 part as known.

The assumption that we already know half the existing species of phænogamous plants is farther opposed by the following considerations. Several thousand species of Monocotyledons and Dycotyledons, and among them tall trees,--(I refer here to my own Expedition),--have been discovered in regions, considerable portions of which had been previously examined by distinguished botanists. The portions of the great continents which have never even been trodden by botanical observers considerably exceed in area those which have been traversed by such travellers, even in a superficial manner. The greatest variety of phænogamous vegetation, _i. e._ the greatest number of species on a given area, is found between the tropics, and in the sub-tropical zones. This last-mentioned consideration renders it so much the more important to remember how almost entirely unacquainted we are, on the New Continent, north of the equator, with the Floras of Oaxaca, Yucatan, Guatimala, Nicaragua, the Isthmus of Panama, Choco, Antioquia, and the Provincia de los Pastos;--and south of the equator, with the Floras of the vast forest region: between the Ucayale, the Rio de la Madera, and the Tocantin (three great tributaries of the Amazons), and with those of Paraguay and the Provincia de los Missiones. In Africa, except in respect to the coasts, we know nothing of the vegetation from 15° north to 20° south latitude; in Asia we are unacquainted with the Floras of the south and south-east of Arabia, where the highlands rise to about 6400 English feet above the level of the sea,--of the countries between the Thian-schan, the Kuenlün, and the Himalaya, all the west part of China, and the greater part of the countries beyond the Ganges. Still more unknown to the botanist are the interior of Borneo, New Guinea, and part of Australia. Farther to the south the number of species undergoes a wonderful diminution, as Joseph Hooker has well and ably shewn from his own observation in his Antarctic Flora. The three islands of which New Zealand consists extend from 34-1/2° to 47-1/4° S. latitude, and as they contain, moreover, snowy mountains of above 8850 English feet elevation, they must include considerable diversity of climate. The Northern Island has been examined with tolerable completeness from the voyage of Banks and Solander to Lesson and the Brothers Cunningham and Colenso, and yet in more than 70 years we have only become acquainted with less than 700 phænogamous species. (Dieffenbach, Travels in New Zealand, 1843, vol. i. p. 419.) The paucity of vegetable corresponds to the paucity of animal species. Joseph Hooker, in his Flora Antarctica, p. 73-75, remarks that the “botany of the densely wooded regions of the Southern Islands of the New Zealand groups and of Fuegia is much more meagre not only than that of similarly clothed regions of Europe, but of islands many degrees nearer to the Northern pole than these are to the Southern one. Iceland, for instance, which is from 8 to 10 degrees farther from the equator than the Auckland and the Campbell Islands, contains certainly five times as many flowering plants. In the Antarctic Flora, under the influence of a cool and moist, but singularly equable climate, great uniformity, arising from paucity of species, is associated with great luxuriance of vegetation. This striking uniformity prevails both at different levels, (the species found on the plains appearing also on the slopes of the mountains), and over vast extents of country, from the south of Chili to Patagonia, and even to Tierra del Fuego, or from lat. 45° to 56°. Compare, on the other hand, in the northern temperate region, the Flora of the South of France, in the latitude of the Chonos Archipelago on the coast of Chili, with the Flora of Argyleshire in Scotland in the latitude of Cape Horn, and how great a difference of species is found; while in the Southern Hemisphere the same types of vegetation pass through many degrees of latitude. Lastly, on Walden Island, in lat. 80-1/2° N., or not ten degrees from the North Pole of the earth, ten species of flowering plants have been collected, while in the southernmost islet of the South Shetlands, though only in lat. 63° S., only a solitary grass was found.” These considerations on the distribution of plants confirm the belief that the great mass of still unobserved, uncollected, and undescribed flowering plants must be sought for in tropical countries, and in the latitudes from 12° to 15° distant from the tropics.

It has appeared to me not unimportant to show the imperfect state of our knowledge in this still little cultivated department of arithmetical botany, and to propound numerical questions in a more distinct and determinate manner than could have been previously done. In all conjectures respecting numerical relations we must seek first for the possibility of deducing the lower or minimum limits; as in a question treated of by me elsewhere, on the proportion of coined gold and silver to the quantity of the precious metal fabricated in other ways; or as in the questions of how many stars, from the 10th to the 12th magnitude, are dispersed over the sky, and how many of the smallest telescopic stars the Milky Way may contain. (John Herschel, Results of Astron. Observ. at the Cape of Good Hope, 1847, p. 381.) We may consider it as established, that if it were possible to know completely and thoroughly by observation all the species belonging to _one_ of the great families of phanerogamous or flowering plants, we should learn thereby at the same time, approximatively, the entire sum of _all_ such plants (including all the families). As, therefore, by the progressive exploration of new countries we progressively and gradually exhaust the remaining unknown species of any of the great families, the previously assigned lowest limit rises gradually higher, and since the forms reciprocally limit each other in conformity with still undiscovered laws of universal organisation, we approach continually nearer to the solution of the great numerical problem of organic life. But is the number of organic forms itself a constant number? Do new vegetable forms spring from the ground after long periods of time, while others become more and more rare, and at last disappear? Geology, by means of her historical monuments of ancient terrestrial life, answers to the latter portion of this question affirmatively. “In the Ancient World,” to use the remark of an eminent naturalist, Link (Abhandl. der Akad. der Wiss. zu Berlin aus dem Jahr 1846, S. 322), “we see characters, now apparently remote and widely separated from each other, associated or crowded together in wondrous forms, as if a greater development and separation awaited a later age in the history of our planet.”

[14] p. 19.--“_If the height of the aerial ocean and its pressure have not always been the same._”

The pressure of the atmosphere has a decided influence on the form and life of plants. From the abundance and importance of their leafy organs provided with porous openings, plants live principally in and through their surfaces; and hence their dependence on the surrounding medium. Animals are dependent rather on internal impulses and stimuli; they originate and maintain their own temperature, and, by means of muscular movement, their own electric currents, and the chemical vital processes which depend on and react upon those currents. A species of skin-respiration is an active and important vital function in plants, and this respiration, in so far as it consists in evaporation, inhalation, and exhalation of fluids, is dependent on the pressure of the atmosphere. Therefore it is that alpine plants are more aromatic, and are hairy and covered with numerous pores. (See my work über die gereizte Muskel- und Nervenfaser, Bd. ii. S. 142-145.) For according to Zoonomic experience, organs become more abundant and more perfect in proportion to the facility with which the conditions necessary for the exercise of their functions are fulfilled,--as I have elsewhere shown. In alpine plants the disturbance of their skin-respiration occasioned by increased atmospheric pressure makes it very difficult for such plants to flourish in the low grounds.

The question whether the mean pressure of the aerial ocean which surrounds our globe has always been the same is quite undecided: we do not even know accurately whether the mean height of the barometer has continued the same at the same place for a century past. According to Poleni’s and Toaldo’s observations, the pressure would have seemed to vary. The correctness of these observations has long been doubted, but the recent researches of Carlini render it almost probable that the mean height of the barometer is diminishing in Milan. Perhaps the phenomenon is a very local one, and dependent on variations in descending atmospheric currents.

[15] p. 20.--“_Palms._”

It is remarkable that of this majestic form of plants,--(some of which rise to more than twice the height of the Royal Palace at Berlin, and to which the Indian Amarasinha gave the characteristic appellation of “Kings among the Grasses”),--up to the time of the death of Linnæus only 15 species were described. The Peruvian travellers Ruiz and Pavon added to these 8 more species. Bonpland and I, in passing over a more extensive range of country from 12° S. lat. to 21° N. lat., described 20 new species of palms, and distinguished as many more, but without being able to obtain complete specimens of their flowers. (Humboldt de distrib. geogr. Plantarum, p. 225-233.) At the present time, 44 years after my return from Mexico, there are from the Old and New World, including the East Indian species brought by Griffith, above 440 regularly described species. The Enumeratio Plantarum of my friend Kunth, published in 1841, had already 356 species.

A few, but only a few species of palms, are, like our Coniferæ, Quercineæ, and Betulineæ, social plants: such are the Mauritia flexuosa, and two species of Chamærops, one of which, the Chamærops humilis, occupies extensive tracts of ground near the Mouth of the Ebro and in Valencia; and the other, C. mocini, discovered by us on the Mexican shore of the Pacific and entirely without prickles, is also a social plant. While some kinds of palms, including Chamærops and Cocos, are littoral or shore-loving trees, there is in the tropics a peculiar group of mountain palms, which if I am not mistaken was entirely unknown previous to my South American travels. Almost all species of the family of palms grow on the plains or low grounds in a mean temperature of between 22° and 24° Reaumur (81°.5 and 86°, Fahr.); rarely ascending so high as 1900 English feet on the declivities of the Andes: but in the mountain palms to which I have alluded, the beautiful Wax-palm (Ceroxylon andicola), the Palmeto of Azufral at the Pass of Quindiu (Oreodoxa frigida), and the reed-like Kunthia montana (Caña de la Vibora) of Pasto, attain elevations between 6400 and 9600 English feet above the level of the sea, where the thermometer often sinks at night as low as 4°.8 and 6° of Reaumur (42°.8 and 45.°5, Fahr.), and the mean temperature scarcely amounts to 11° Reaumur, or 56°.8 Fahrenheit. These Alpine Palms grow among Nut trees, yew-leaved species of Podocarpus and Oaks (Quercus granatensis). I have determined by exact barometrical measurement the upper and lower limits of the range of the Wax-Palm. We first began to find it on the eastern declivity of Andes of Quindiu, at the height of 7440 (about 7930 English) feet above the level of the sea, and it extended upwards as far as the Garita del Paramo and los Volcancitos, or to 9100 (almost 9700 English) feet: several years after my departure from the country the distinguished botanist Don Jose Caldas, who had been long our companion amidst the mountains of New Granada, and who afterwards fell a victim to Spanish party hatred, found three species of palms growing in the Paramo de Guanacos very near the limits of perpetual snow; therefore probably at an elevation of more than 13000 (13855 English) feet. (Semanario de Santa Fé de Bogotá, 1809, No. 21, p, 163.) Even beyond the tropics, in the latitude of 28° North, the Chamærops martiana reaches on the sub-Himalayan mountains a height of 5000 English feet. (Wallich, Plantæ Asiaticæ, Vol. iii. Tab. 211.)

If we look for the extreme geographical limits of palms, (which are also the extreme climatic limits in all the species which inhabit localities but little raised above the level of the sea), we see some, as the date-palm, the Chamærops humilis, C. palmetto, and the Areca sapida of New Zealand, advance far into the temperate zones of either hemisphere, into regions where the mean temperature of the year hardly equals 11°.2 and 12°.5 Reaumur (57°.2, and 60°.2 Fahrenheit). If we form a series of cultivated plants or trees, placed in order of succession according to the degree of heat they require, and beginning with the maximum, we have Cacao, Indigo, Plantains, Coffee, Cotton, Date-palms, Orange and Lemon Trees, Olives, Sweet Chestnuts, and Vines. In Europe, date-palms (introduced, not indigenous) grow mingled with Chamærops humilis in the parallels of 43-1/2° and 44°, as on the Genoese Rivera del Ponente, near Bordighera, between Monaco and San Stefano, where there is an assemblage of more than 4000 palm-stems; and in Dalmatia round Spalatro. It is remarkable that Chamærops humilis is abundant both at Nice and in Sardinia, and yet is not found in the island of Corsica which lies between those localities. In the New Continent, the Chamærops palmetto, which is sometimes above 40 English feet high, only advances as far North as 34° latitude, a difference sufficiently explained by the inflexions of the isothermal lines. In the Southern hemisphere, in New Holland, palms, of which there are very few, (six or seven species) only advance to 34° of latitude (see Robert Brown’s general remarks on the Botany of Terra Australis, p. 45); and in New Zealand, where Sir Joseph Banks first saw an Areca palm, they reach the 38th parallel. In Africa, which, quite contrary to the ancient and still widely prevailing belief, is poor in species of palms, only one palm, the Hyphæne coriacea, advances to Port Natal in 30° latitude. The continent of South America presents almost the same limits in respect to latitude. On the eastern side of the Andes, in the Pampas of Buenos Ayres and in the Cis-Plata province, palms extend, according to Auguste de St.-Hilaire, to 34° and 35° S. latitude. This is also the latitude to which on the western side of the Andes the Coco de Chile (our Jubæa spectabilis?), the only Chilian palm, extends, according to Claude Gay, being as far as the banks of the Rio Maule. (See also Darwin’s Journal, edition of 1845, p. 244 and 256).

I will here introduce some detached remarks which I wrote in March, 1801, on board the ship in which we were sailing from the palmy shores of the mouth of the Rio Sinu, west of Darien, to Cartagena de las Indias.

“We have now, in the course of the two years which we have spent in South America, seen 27 different species of palms. How many must Commerson, Thunberg, Banks, Solander, the two Forsters, Adanson, and Sonnerat, have observed in their distant voyages! Yet, at the present moment, when I write these lines, our systems of botany do not include more than from 14 to 18 systematically described species. In truth, the difficulty of procuring the flowers of palms is greater than can readily be imagined. We have felt it so much the more from having especially directed our attention to Palms, Grasses, Cyperaceæ, Juncaceæ, Cryptogamous Plants, and such other objects as have been least studied hitherto. Most species of palms flower only once a year, in the neighbourhood of the Equator in the months of January and February. But how often is it impossible for travellers to be precisely at that season in places where palms are principally found. In many species of palms the flowers last only so few days that one almost always arrives too late, and finds the fertilization completed and the male blossoms gone. Frequently only three or four species of palms are found in areas of 2000 square German geographical miles (3200 English geographical square miles). How is it possible during the short flowering season to visit the different places where palms abound: the Missions on the Rio Caroni, the Morichales at the mouth of the Orinoco, the valley of Caura and Erevato, the banks of the Atabapo and the Rio Negro, and the side of the Duida Mountain? Add to this the difficulty of reaching the flowers, when, in the dense forests, or on the swampy river banks, (as on the Temi and Tuamini), one sees them hanging from stems above 60 feet high, and armed with formidable spines. A traveller, when preparing to leave Europe on an expedition in which natural history is one of his leading objects, flatters himself with the thoughts of shears or curved blades fastened to long poles, with which he imagines he will be able to reach and cut down whatever he desires; he dreams, too, of native boys, who, with a cord fastened to their two feet, are to climb up the highest trees at his bidding. But, alas! very few of these fancies are ever realised; the great height of the blossoms renders the poles useless; and in the missions established on the banks of the rivers of Guiana, the traveller finds himself among Indians whose poverty, stoicism, and uncultivated state, renders them so rich, and so free from wants of every kind, that neither money nor other presents that can be made to them will induce them to turn three steps out of their path. This insurmountable apathy is the more provoking to a European, because he sees the same people climb with inconceivable agility wherever their own fancies lead them; for example, when they wish to catch a parrot, or an iguana, or a monkey, which having been wounded by their arrows saves himself from falling by holding on to the branches with his prehensile tail. Even at the Havannah we met with a similar disappointment. We were there in the month of January, and saw all the trees of the Palma Real (our Oreodoxa Regia), in the immediate vicinity of the city and on the public walks, adorned with snow-white blossoms. For several days we offered the negro boys whom we met in the streets of Regla and Guanavacoa two piastres for a single bunch of the blossoms which we wanted, but in vain! Between the tropics men are indisposed to laborious exertion, unless compelled by constraint or by extreme destitution. The botanists and artists of the Royal Spanish Commission for researches in Natural History, under the direction of Count Jaruco y Mopor (Estevez, Boldo, Guio, and Echeveria),--acknowledged to us that during several years they had not been able to obtain these flowers for examination. These difficulties sufficiently explain what would have been incomprehensible to me before my voyage, namely, that although during our two years’ stay up to the present time, we have, indeed, discovered more than 20 different species of palms, we have as yet been only able to describe systematically 12. How interesting a work might be produced by a traveller in South America who should occupy himself exclusively with the study of palms, and should make drawings of the spathe, spadix, inflorescence, and fruit, all of the size of nature!” (I wrote this many years before the Brazilian travels of Martius and Spix, and the admirable and excellent work of Martius on Palms.) “There is considerable uniformity in the shape of the leaves of palms; they are generally either pinnate (feathery, or divided like the plume of a feather);--or else palmate or palmo-digitate (of a fan-like form); the leaf-stalk (petiolus), is in some species without spines, in others sharply toothed (serrato-spinosus). The form of the leaf in Caryota urens and Martinezia caryotifolia, (which we saw on the banks of the Orinoco and Atabapo, and again in the Andes, at the pass of Quindiu, 3000 Fr. (3197 English) feet above the level of the sea), is exceptional and almost unique among palms, as is the form of the leaf of the Gingko among trees. The port and physiognomy of palms have a grandeur of character very difficult to convey by words. The stem, shaft, or caudex, is generally simple and undivided, but in extremely rare exceptions divides into branches in the manner of the Dracænas, as in Cucifera thebaica (the Doum-palm), and Hyphæne coriacea. It is sometimes disproportionately thick (as in Corozo del Sinu, our Alfonsia oleifera); sometimes feeble as a reed (as in Piritu, Kunthia montana, and the Mexican Corypha nana); sometimes swelling towards the base (as in Cocos); sometimes smooth, and sometimes scaly (Palma de covija o de sombrero, in the Llanos); sometimes armed with spines (as Corozo de Cumana and Macanilla de Caripe), the long spines being distributed with much regularity in concentric rings.

“Characteristic differences are also furnished in some species by roots which, springing from the stem at about a foot or a foot and a half above the ground, either raise the stem as it were upon a scaffolding, or surround it with thick buttresses. I have seen Viverras, and even very small monkeys, pass underneath this kind of scaffolding formed by the roots of the Caryota. Often the shaft or stem is swollen only in the middle, being more slender above and below, as in the Palma Real of the Island of Cuba. The leaves are sometimes of a dark and shining green (as in the Mauritia and the Cocoa nut palm); sometimes of a silvery white on the under side (as in the slender Fan-palm, Corypha miraguama, which we found in the Harbour of Trinidad de Cuba). Sometimes the middle of the fan or palmate leaf is ornamented with concentric yellowish or bluish stripes like a peacock’s tail; as in the thorny Mauritia which Bonpland discovered on the banks of the Rio Atabapo.