Influence of Temperature on Vegetation—Vegetation varies with the
Latitude and Height above the Sea—Geographical Distribution of Land
Plants—Distribution of Marine Plants—Corallines, Shell-fish, Reptiles,
Insects, Birds, and Quadrupeds—Varieties of Mankind, yet identity of
Species.
THE gradual decrease of temperature in the air and in the earth, from
the equator to the poles, is clearly indicated by its influence on
vegetation. In the valleys of the torrid zone, where the mean annual
temperature is very high, and where there is abundance of light and
moisture, nature adorns the soil with all the luxuriance of perpetual
summer. The palm, the bombax ceiba, and a variety of magnificent trees,
tower to the height of 150 or 200 feet above the banana, the bamboo, the
arborescent fern, and numberless other tropical productions, so
interlaced by creeping and parasitical plants, as often to present an
impenetrable barrier. But the richness of vegetation gradually
diminishes with the temperature; the splendour of the tropical forest is
succeeded by the regions of the vine and olive; these again yield to the
verdant meadows of more temperate climes; then follow the birch and the
pine, which probably owe their existence in very high latitudes more to
the warmth of the soil than to that of the air. But even these enduring
plants become dwarfish shrubs, till a verdant carpet of mosses and
lichens, enamelled with flowers, exhibits the last sign of vegetable
life during the short but fervid summers at the polar regions. Such is
the effect of cold and diminished light on the vegetable kingdom, that
the number of species growing under the equator and in the northern
latitudes of 45° and 68° are in the proportion of the numbers 12, 4, and
1. Notwithstanding the remarkable difference between a tropical and
polar flora, light and moisture seem to be almost the only requisites
for vegetation, since neither heat, cold, nor even comparative darkness,
absolutely destroy the fertility of nature. In salt plains and sandy
deserts alone hopeless barrenness prevails. Plants grow on the borders
of hot springs: they form the oases wherever moisture exists among the
burning sands of Africa; they are found in caverns almost void of light,
though generally blanched and feeble. The ocean teems with vegetation.
The snow itself not only produces a red lichen, discovered by Saussure
in the frozen declivities of the Alps, found in abundance by the author
crossing the Col de Bonhomme from Savoy to Piedmont, and by the polar
navigators in the Arctic regions, but it affords shelter to the
productions of these inhospitable climes against the piercing winds that
sweep over fields of everlasting ice. Those undaunted mariners narrate
that under this cold defence plants spring up, dissolve the snow a few
inches round, and the part above, being again quickly frozen into a
transparent sheet of ice, admits the sun’s rays, which warm and cherish
the plants in this natural hothouse, till the returning summer renders
such protection unnecessary.
The chemical action of light is, however, absolutely requisite for the
growth of plants which derive their principal nourishment from the
atmosphere. They consume the carbonic acid gas, nitrogen, aqueous
vapour, and ammonia it contains; but it is the chemical agency of light
that enables them to absorb, decompose, and consolidate these substances
into wood, leaves, flowers, and fruit. The atmosphere would soon be
deprived of these elements of vegetable life were they not perpetually
supplied by the animal creation; while, in return, plants decompose the
moisture they imbibe, and, having assimilated the carbonic acid gas,
they exhale oxygen for the maintenance of the animated creation, and
thus preserve a just equilibrium. Hence it is the combined and powerful
influences of the whole solar beams that give such brilliancy to the
tropical forests, while, with their decreasing energy in the higher
latitudes, vegetation becomes less vigorous. On that account it is vain
to expect that the fruit and flowers raised in our hothouses can ever
have the flavour, perfume, or colouring equal to that which they acquire
from the vivid light of their native skies.
By far the greater number of the known species of plants are indigenous
in equinoctial America; Europe contains about half the number; Asia,
with its islands, somewhat less than Europe; Australia, with the islands
in the Pacific, still less; and in Africa there are fewer known
vegetable productions than in any part of the globe of equal extent, for
that rich and luxuriant region discovered by Dr. Livingstone has yet to
be explored botanically. Very few social plants, such as grasses and
heaths that cover large tracts of land, are to be found between the
tropics, except on the sea-coasts and elevated plains. Some exceptions
to this, however, are to be met with in the jungles of the Deccan, &c.
In the equatorial regions, where the heat is always great, the
distribution of plants depends upon the mean annual temperature; whereas
in temperate zones the distribution is regulated in some degree by the
summer heat. Some plants require a gentle heat of long continuance,
others flourish most where the extremes of heat and cold are greater.
The range of wheat is very great; it may be cultivated as far north as
the 60th degree of latitude; but in the torrid zone it will seldom form
an ear below an elevation of 4500 feet above the level of the sea from
exuberance of vegetation; nor will it ripen generally above the height
of 12,000 feet; in Tibet it ripens at a still greater elevation. Colonel
Sykes states that in the Deccan wheat thrives as low as 1800 feet above
the sea. The best wines are produced between the 30th and 45th degrees
of north latitude. With regard to the vegetable kingdom, elevation is
equivalent to latitude as far as temperature is concerned. In ascending
the mountains of the torrid zone, the richness of the tropical
vegetation diminishes with the height; a succession of plants similar
to, though not identical with, those found in latitudes of corresponding
mean temperature takes place; the lofty forests by degrees lose their
splendour; stunted shrubs succeed; till at last the progress of the
lichen is checked by perpetual snow. On the volcano of Teneriffe there
are five successive zones, each producing distinct families of plants.
The first is the region of vines, the next that of laurels; these are
followed by the region of pines, of Ericas or heaths, of grass; the
whole covering the declivity of the peak through an extent of 11,200
feet of perpendicular height.
Near the equator oaks flourish at the height of 9200 feet above the sea;
and, on the lofty range of the Himalaya, the primula, the convallaria,
and the veronica flower, but not the primrose, the lily of the valley,
or the veronica, which adorn our meadows; for, although the herbarium
collected by Moorcroft, on his route from Neetee to Daba and Gartope in
Chinese Tartary, at elevations as high or even higher than Mont Blanc,
abound in Alpine and European genera, the species are universally
different, with the single exception of the Rhodiola rosea, which is
identical with the species that blooms in Scotland. It is not in this
instance alone that similarity of climate obtains without identity of
productions; throughout the whole globe a certain analogy both of
structure and appearance is frequently discovered between plants under
corresponding circumstances which are yet specifically different. It is
even said that a difference of 25° of latitude occasions a total change,
not only of vegetable productions, but of organised beings. Certain it
is that each separate region both of land and water, from the frozen
shores of the polar circles to the burning regions of the torrid zone,
possesses a flora peculiarly its own. The whole globe has been divided
by physical geographers into various botanical districts, differing
almost entirely in their specific vegetable productions, the limits of
which are most decided when they are separated by a wide expanse of
ocean, mountain chains, sandy deserts, salt plains, or internal seas. A
considerable number of plants are common to the northern regions of
Asia, Europe, and America, where the continents almost unite; but, in
approaching the south, the floras of these three great divisions of the
globe differ more and more even in the same parallels of latitude, which
shows that temperature alone is not the cause of the almost complete
diversity of species that everywhere prevails. The floras of China,
Siberia, Tartary, of the European district including central Europe and
the coast of the Mediterranean, and the Oriental region comprising the
countries round the Black and Caspian Seas, all differ in specific
character. Only twenty-four species were found by MM. Humboldt and
Bonpland in Equinoctial America identical with those of the Old World;
and Dr. Robert Brown not only found that a peculiar vegetation exists in
Australia between the 33rd and 35th parallels of south latitude, but
that at the eastern and western extremities of these parallels not one
species is common to both, and that certain genera also are almost
entirely confined to these spots. The number of species common to
Australia and Europe are only 166 out of 4100, and probably some of
these have been conveyed thither by the colonists; but the greater part
of that continent is still unexplored. However, this proportion exceeds
what has hitherto been observed in southern Africa, and, from what has
been already stated, the proportion of European species in Equinoctial
America is still less.
Islands partake of the vegetation of the nearest continents; but, when
very remote from land, their floras are altogether peculiar. The
Aleutian Islands, extending between Asia and America, partake of the
vegetation of the northern parts of both continents, and may have served
as a chain of communication. In Madeira and Teneriffe, the plants of
Portugal, Spain, the Azores, and of the northern coast of Africa, are
found; and the Canaries contain a great number of plants belonging to
the African coast. But each of these islands possesses a flora that
exists nowhere else; and St. Helena, standing alone in the midst of the
Atlantic Ocean, produces only two or three species of plants recognised
as belonging to any other part of the world.
It appears from the investigations of M. de Humboldt that between the
tropics the plants, such as grasses and palms, which have only one
seed-lobe, are to the tribe which have two seed-lobes, like most of the
European species, in the proportion of one to four; in the temperate
zones they are as one to six; and in the Arctic regions, where mosses
and lichens, which form the lowest order of the vegetable creation,
abound, the proportion is as one to two. Annuals with one and two
seed-lobes, in the temperate zones, amount to one-sixth of the whole,
omitting the cryptogamia (N. 218); in the torrid zone they scarcely form
one-twentieth, and in Lapland one-thirtieth part. In approaching the
equator the ligneous exceed the number of herbaceous plants; in America
there are 120 different species of forest trees, whereas in the same
latitudes in Europe only 34 are to be found.
Similar laws regulate the distribution of marine plants. Groups of algæ,
or marine plants, affect particular temperatures or zones of latitude
and different depths, though some few genera prevail throughout the
ocean. The polar Atlantic basin to the 40th degree of north latitude
presents a well-defined vegetation. The West India seas, including the
Gulf of Mexico, the eastern coast of South America, the Indian Ocean and
its gulfs, the shores of New Holland, and the neighbouring islands, have
each their distinct species. The Mediterranean possesses a vegetation
peculiar to itself, extending to the Black Sea; and the species of
marine plants on the coast of Syria and in the port of Alexandria differ
almost entirely from those of Suez and the Red Sea. It is observed that
shallow seas have a different set of plants from such as are deeper and
colder; and, unlike terrestrial vegetation, the algæ are more numerous
in the mean latitudes than either towards the equator or the poles. They
vary also with the depth: completely different kinds affect different
depths, their seeds being of such specific gravity as to remain and
germinate where the parent plant grew. The quantity of algæ in that
accumulation known as the sargassa or grassy sea is so great, that the
early navigators, Columbus and Lerius, compared it to extensively
inundated meadows: it impeded their ships, and alarmed their sailors. It
is in the North Atlantic, a little to the west of the meridian of Fayal,
one of the Azores, between the 25th and 36th parallels of latitude. A
smaller bank lies between the 22nd and 26th degrees of north latitude,
about 80 leagues west of the meridian of the Bahama Islands. These
masses chiefly consist of one or two species of sargassa, the most
extensive genus of the order Fucoideæ.
Some of the seaweeds grow to enormous lengths, and all are highly
coloured, though many of them must grow in deep water. Light, however,
may not be the only principle on which the colour of vegetables depends,
since Baron Humboldt met with green plants growing in complete darkness
in one of the mines at Freyberg.
In the dark and tranquil caves of the ocean, on the shores alternately
covered and deserted by the restless waves, on the lofty mountain and
extended plain, in the chilly regions of the north, and in the genial
warmth of the south, specific diversity is a general law of the
vegetable kingdom, which cannot be accounted for by diversity of
climate; and yet the similarity, though not identity, of species is
such, under the same isothermal lines, that if the number of species
belonging to one of the great families of plants be known in any part of
the globe, the whole number of the flowering or more perfect plants, and
also the number of species composing the other vegetable families, may
be estimated with considerable accuracy.
Various opinions have been formed on the original or primitive
distribution of plants over the face of the globe; but, since botanical
geography has become a science, the phenomena observed have led to the
conclusion that vegetable creation must have taken place in a number of
distinctly different centres, as the islands and continents rose above
the ocean, each of which was the original seat of a certain number of
peculiar species which at first grew there and nowhere else. Heaths are
exclusively confined to the Old World; and no indigenous rose-tree has
ever been seen in the New, the whole southern hemisphere being destitute
of that beautiful and fragrant plant. But this is still more confirmed
by multitudes of particular plants, having an entirely local and
insulated existence, growing spontaneously in some particular spot, and
in no other place: for example, the cedar of Lebanon, which grows
indigenously on that mountain, and in no other part of the world. On the
other hand, as there can be no doubt that many races of plants have been
extinguished, Sir John Herschel thinks it possible that these solitary
instances may be the last surviving remnants of the same group
universally disseminated, but in course of extinction, or that perhaps
two processes may be going on at the same time:—“Some groups may be
spreading from their foci, others retreating to their last holds.”
The same laws obtain in the distribution of the animal creation. Even
the microscopic existences, which seem to be the most widely spread,
have their specific localities; and the zoophyte (N. 219), occupying the
next lowest place in animated nature, is widely scattered through the
seas of the torrid zone, each species being confined to the district and
depth best suited to its wants. Mollusks, or the animals of shells,
decrease in size and beauty with their distance from the equator; and
not only each sea and every basin of the ocean, but each depth, is
inhabited by its peculiar tribe of fish. Indeed, MM. Peron and Le Sueur
assert that, among the many thousands of marine animals which they had
examined, there is not a single animal of the southern regions which is
not distinguishable by essential characters from the analogous species
in the northern seas.
Reptiles are not exempt from the general law. The saurian (N. 220)
tribes of the four quarters of the globe differ in species; and,
although warm countries abound in venomous snakes, they are specifically
different in different localities, and decrease both in numbers and in
the virulence of their poison with decrease of temperature. The
dispersion of insects necessarily follows that of the vegetables which
supply their food; and in general it is observed that each kind of plant
is peopled by its peculiar inhabitants. Each species of bird has its
peculiar haunt, notwithstanding the locomotive powers of the winged
tribes. The emu is confined to Australia, the condor to the Andes and
their declivities, and the bearded vulture or lemmergeyer to the Alps.
Some birds, like the common sparrow, have a wide range; but those met
with in every country are few in number. Quadrupeds are distributed in
the same manner wherever man has not interfered. Such as are indigenous
in one country are not the same with their congeners in another; and,
with the exception of some kind of bats, no mammiferous animal is
indigenous in the Polynesian Archipelago, nor in any of the islands on
the borders of the central part of the Pacific.
In reviewing the infinite variety of organised beings that people the
surface of the globe, nothing is more remarkable than the distinctions
which characterise the different tribes of mankind, from the ebony skin
of the torrid zone to the fair and ruddy complexion of the
Scandinavian—a difference which existed in the earliest recorded times,
since the African is represented in the sacred writings to have been as
black as he is at the present day, and the most ancient Egyptian
paintings confirm that truth; yet it appears, from a comparison of the
principal circumstances relating to the animal economy or physical
character of the various tribes of mankind, that the different races are
identical in species. Many attempts have been made to trace the various
tribes back to a common origin, by collating the numerous languages
which are or have been spoken. Some classes of these have few or no
words in common, yet exhibit a remarkable analogy in the laws of their
grammatical construction. The languages spoken by the native American
nations afford examples of these; indeed, the refinement in the
grammatical construction of the tongues of the American savages leads to
the belief that they must originally have been spoken by a much more
civilised class of mankind. Some tongues have little or no resemblance
in structure, though they correspond extensively in their vocabularies,
as the Syrian dialects. In all these cases it may be inferred that the
nations speaking the languages in question descended from the same
stock; but the probability of a common origin is much greater in the
Indo-European nations, whose languages, such as the Sanscrit, Greek,
Latin, German, &c., have an affinity both in structure and
correspondence of vocables. In many tongues not the smallest resemblance
can be traced; length of time, however, may have obliterated original
identity; but so many ages have passed before the subject became a
study, and so many languages have worn out of use, that it may be
doubted whether any satisfactory result will ever be arrived at with
regard to the original speech of mankind.