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.