The publication of the volume, the title of which appears below,[3] is
to be regarded as additional evidence that the tendency of science in
the present day is towards wider and more comprehensive
generalisations. Many readers who may be more or less familiar with
certain species or even families of plants, will hardly have prepared
themselves for a view of the phytology of a quarter of the globe, such
as is given in outline in the interesting work now before us. The
subject is one that has been largely investigated within the past
twenty years, as may be seen in the records of the British
Association, in the transactions of learned societies, and in the
writings of numerous observers on the continent. Attempt after attempt
has been made to explain the causes of the variations and effects of
climate, their influence on vegetation, the appearance of certain
floras in localities where they might be least expected, and to
separate the natural and regular from the accidental. Different
countries have been examined and compared with each other, and many of
the differences accounted for; and in Mr Henfrey's volume we have an
acceptable _résumé_ of these various researches.
It becomes necessary, first of all, to study the influences--whether
general or special--which affect the distribution of vegetation; to
inquire into those freaks or aberrations of nature which favour in one
place the production of plants that will not grow in another, under
apparently similar circumstances; and why similar plants are found in
places widely separated. Oranges will ripen on one side the Alps, but
not on the other; grapes scarcely come to perfection out of doors in
England, while on the other side of the Channel they ripen by
thousands of acres; and several fruits which fail in our northern
counties, are grown without difficulty in Denmark in the open air.
Investigation soon shewed that temperature alone, mere heat and cold,
was insufficient to account for the phenomena; but that moisture and
dryness, the prevalence of certain winds, the chemical and physical
conditions of soil, and the constitution of the plants themselves,
would have to be considered in a proper inquiry into the subject.
Here we must notice a fact which has proved of essential service in
the study of botanical geography--namely, the discovery 'that there is
some law presiding over the distribution of plants which causes the
appearance of particular species arbitrarily--if we may so say it--in
particular places;' from which, the conclusion has been arrived at,
'that countries have become populated with plants partly by the
spreading of some special kinds from centres within those countries
where they were originally exclusively created; and while these have
spread outward into the neighbouring regions, colonists from like
centres lying in the surrounding countries have invaded and become
intermingled with the indigenous inhabitants.'
Looking at the effect of climate on vegetation, we find that as we
proceed from the north towards the south, the number and luxuriance of
plants increase in a remarkable degree, and the same result is
observable in altitude as in latitude. 'Step by step,' writes Mr
Henfrey, 'as the land rises in any mountain region, the vegetation
assumes, more and more, a polar character; and in the mountains of the
tropics, a succession of stages has been distinguished, corresponding
in the general peculiarities of the plants which clothe them, to
tracts extending horizontally, in succession, on the sea-level, from
the base of these mountains to the frozen regions within the arctic
and antarctic circles. Increase of elevation is accompanied by an
alteration of climate, bringing with it a set of conditions analogous
to those prevailing at certain distances further from the sun.
Ascending the Peak of Teneriffe, a series of regions are traversed,
one above another, displaying with the approach to the summit a
continually closer approximation in character to the polar regions,
till the traveller who left the palm, the cactus, and the thousand
varied forms of tropical vegetation at the foot, finds himself at last
among the stunted shrubs and scaly lichens, the borderers who hold the
outposts on the limits of the eternal snow.'
It might be expected that places on the same parallel of latitude
would be equal in temperature; but on tracing out the distribution of
heat over the globe, and laying it down in what are called
_isothermal_ lines on a map, most striking deviations are found to
exist, and the contour of the lines is anything but regular. The line
of greatest cold, for example, which leaves the eastern coast of
Labrador at about the 54th degree of latitude, rises six degrees as it
approaches Greenland, and strikes the coast of Lapland a little above
the 70th degree, or sixteen degrees nearer the pole than at its
starting-point--thus shewing that the northern parts of Europe have a
more genial climate than those of America. The line then curves
fifteen degrees to the south across Siberia, rises again on the
western coast of America, and falls once more as it advances towards
the east. Again, 'the isotherms of Canada pass through Iceland, across
about the middle of Norway and Sweden, St Petersburg and Kamtschatka.
Those of New York through the north of Ireland and England, twelve
degrees further north, North and Central Germany, and the Crimea. That
which leaves the United States at about 36 degrees north latitude,
crosses Southern Europe from the north of Spain to the Adriatic in a
tolerably straight line, some eight degrees further north, and then
falls south again, where the influence of the north-east polar current
is more felt, in Greece and Turkey.'
But although these are marked as lines of equal heat, it is only in
the average temperature that the equality consists; and it is clear
that a country with 80 degrees of summer heat and 20 of winter cold,
would have a very different climate from another with 60 and 40 as the
highest and lowest degrees of temperature, although the mean of the
two would be the same. And herein we have an explanation of what at
first sight appear to be anomalies: we know, for instance, 'that
plants will flourish perennially in the British isles which are killed
by the frosts of winter in places lying considerably to the south upon
the continent; thus the laurel, that bears our winters steadily in
Ireland and the west of England, and is only affected by very severe
frosts in our eastern counties, is killed by the winters of Berlin,
equally fatal to the myrtle, the fuchsia, and a host of other shrubs
which attain considerable age and size in the western portions of the
British isles. Again, Canada, which lies south of Paris, has the
climate of Drontheim, in Norway; while at New York, lying in the
latitude of Naples, the flowers open simultaneously with those of
Upsala, in Sweden. Moreover, those very countries suffering so severe
a winter's cold, enjoy a summer's heat far exceeding ours, since the
snow lies for months on parts of Germany which yet receive sufficient
heat in summer to ripen the grape and Indian corn.'
The principal modifying causes are winds and water. Islands, and
countries bordering on the ocean, have a much more equable climate
than those which lie in the interior of continents, and will have a
greater prevalence of moist south-westerly winds. The average annual
quantity of rain in the British islands is from 28 to 30 inches; on
the continent, it is less; the fall in Holland is estimated at 26
inches, and in Denmark and North Germany, at 20 inches--the greatest
fall occurring in summer and autumn, as in England. Then with respect
to winds, we find those from the west most prevalent over what Mr
Henfrey distinguishes as the north European plain, as is the case in
our country. 'The west wind blows more frequently in England than in
Denmark, more there than in Russia. The predominance is most marked in
summer; in the winter, the easterly winds are almost as frequent as
the westerly upon the continent, which is not true of the British
isles.' Sometimes, however, the south-westerly winds, which bring our
genial April showers, continue to arrive with their watery burden
until late in the summer, to the detriment or destruction of
grain-crops; and yet this same wind, losing its excessive moisture as
it sweeps onward over the continent, is highly favourable to the
husbandman in Southern Russia. The years 1816 and 1817 were cases in
point.
The meteorology of Russia affords some striking contrasts: the yearly
rain-fall in St Petersburg is 21 inches, 'and the westerly winds are
most prevalent, although not to the same extent as in Western Europe;
they are also predominant in Moscow and Kasan. In the southern
steppes, it is stated that the average of four years has given only 6
inches fall of rain, occurring in 47 days of the year; but the
irregularity is so great, that single years gave 59, 35, 39, and 53
rainy days. In 1832-3, twenty months elapsed without rain, and in some
years the quantity is only one-tenth of that which falls in wet years.
In the summer, there is no dew, and the ground dries up and cracks,
the plants withering up: 1841, not considered as a dry year, gave only
8-1/2 inches of rain; but in 1831, one of the wettest, the moisture
interfered with agriculture more than the drought does, saturating the
soil, which rests on a deep impermeable clayey formation.' In April
and May, when the snows melt, the steppe is a vast sea of mud, liable
to be hardened by occasional frosts, until, as the season advances,
myriads of crocuses, tulips, and hyacinths, cover the soil, which
perhaps a few days later will be hidden by north-east snowstorms, or
drenched by gales from the north-west. No rain falls for two months
after the middle of June, the luxuriant herbage withers more rapidly
than it grew, and, except in a few spots near the streams, the steppe
becomes a black, arid waste. Yet in some parts of these regions the
vegetation is extraordinary: 'the wormwoods and thistles grow to a
size unknown in the west of Europe; it is said that the thistle-bush,
found where these abound, is tall enough to hide a Cossack horseman.
The natives call all these rank weeds, useless for pasture, _burian_,
and, with the dry dung of the flocks, this constitutes all the fuel
they possess. One curious plant of the thistle tribe has attracted the
notice of most travellers--the wind-witch, as it is called by the
German colonists, or leap-the-field, as the Russian name may be
translated. It forms a large globular mass of light wiry branches
interlaced together, and in autumn decays off at the root, the upper
part drying up. It is then at the mercy of the autumn blast, and it is
said that thousands may sometimes be seen coursing over the plain,
rolling, dancing, and leaping over the slight inequalities, often
looking at a distance like a troop of wild horses. It is not uncommon
for twenty or thirty to become entangled into a mass, and then roll
away, as Mr Kohl says, "like a huge giant in his seven-league boots."
Thousands of them are annually blown into the Black Sea, and here,
once in contact with water, in an instant lose the fantastic grace
belonging to their dry, unsubstantial texture.'
Any one who has seen the feather-like seeds of thistles and dandelions
floating about in the air, will have little difficulty in
comprehending the effect of winds on the distribution of vegetation.
Such seeds, as Mr Henfrey observes, might readily be carried across
Europe by a powerful autumn gale, blowing steadily in one direction.
In physiological language, they belong to the _sporadic_, not to the
_endemic_ class, of which a remarkable instance is afforded in the
flea-bane (_Erigeron canadensis_), a plant which, having found its way
to this side the Atlantic only since the discovery of America, is now
a common weed on the continent of Europe. Running streams and ocean
currents also transport seeds from one locality to another. The
gulf-stream, as is well known, carries occasionally branches of trees
to the north coast of Scotland and Norway; and 'Mr Brown found that
six hundred plants collected about the river Zaire, in Africa,
included thirteen species, natives also of Guyana and Brazil. These
species mostly occurred near the mouth of the Zaire, and were of such
kind as produced fruits capable of resisting external agencies for a
long time.' Then, again, the agency of birds, of quadrupeds, and of
man, in the distribution of seeds and plants, is too important to be
overlooked, as Sir Charles Lyell has ably shewn in his _Principles of
Geology_; and there is 'a certain number of plants which seem to
accompany man wherever he goes, and to flourish best in his vicinity.
Thus, the docks, the goosefoots, the nettle, the chickweed, mallows,
and many other common weeds, seem to be universal, though unwelcome
companions to man--dogging his footsteps, affording by their presence,
even in now deserted districts, an almost certain index of the former
residence of human beings on the spot.'
From an examination of the causes affecting distribution, Mr Henfrey
passes to a survey of the characteristics of the countries of Europe,
from north to south--from the peninsula of Scandinavia to those of
Spain, Italy, and Greece. The remarkable contrast is pointed out
between the climate and cultivation of the east and west sides of the
mountains of Sweden and Norway. Barley ripens as far north as the 70th
degree, in latitudes whose mean temperature is below the
freezing-point; while in Switzerland, corn ceases to ripen at 9
degrees above the same point, and in the plateaux of South America, at
22-1/2 degrees--a fact which goes to shew, 'that the growth of grain
is much more dependent on the summer temperature than on the annual
mean. The long summer days of the polar regions afford a very brief,
but a comparatively exalted summer heat.' It is, however, only the
barley which ventures so far north: the limit of rye is 67 degrees, of
oats, 65 degrees, of wheat, 64 degrees, on the west side of the
peninsula, and from 1 to 2 degrees less on the east. In Southern
Norway, the spruce-fir ceases to grow beyond the line of 2900 feet
above the sea-level; while in Switzerland, it is commonly met with at
the height of 5500 feet, and in some situations, 7000; shewing that
the influences which affect the growth of grain do not similarly
affect that of trees--proximity of the sea decreases the summer
temperature. Again: 'In Scandinavia the tree-limit is indicated by the
birch; in the Alps, by firs. The two lower mountain zones of the
Alps, the regions of the beech and the chestnut, do not exist in the
Scandinavian mountains. Compared with the climate and tree-limits, the
cultivation of corn does not go so high in the Alps as it does toward
the north; for it ceases about with the beech in the Alps, and grazing
is the regular pursuit in the region of firs; while in Scandinavia,
the beech only goes to 59 degrees, and corn-culture to 70
degrees--that is, as far as the conifers. Corn succeeds in the latter
under a mean temperature below the freezing-point, while in the Alps
it ceases at 41 degrees Fahrenheit. The cause of this is the hot
though short summer of the north. The Alps have maize and the vine,
which will not grow around the Scandinavian mountains; the meadows are
throughout richer in the Alps, and grazing is therefore much more
extensively pursued.'
The peculiarities and comparisons afforded by other countries, are not
less interesting than those we have selected, and we might multiply
instances, if space permitted. Enough, however, have been adduced to
shew that the mode of accounting for differences of vegetation is so
far satisfactory, that it appears to be in perfect accordance with
discoverable natural laws; and it is no longer a surprise or mystery
to find plants of Southern Russia and of Asia Minor on the high
table-lands of Spain; or that the effects of an unvarying temperature,
as at Quito, in the table-land of Peru, are to cause the culture of
wheat to cease at the mean temperature of Milan, and woods to
disappear at the mean of Penzance. A few remarks respecting our own
country is all that we can now find room for.
Including snow-falls, the number of rainy days in Dublin in a year is
208, in London, 178, while in Copenhagen it is not more than 134. The
number of British plants indigenous or naturalised is from 1400 to
1500, comprising mostly the vegetation of Central Europe, but
including specimens from Scandinavia and the Pyrenees. The highest
point at which grain has been known to grow, is 1600 feet above the
sea-level, at the outlet of Loch Collater, in the Highlands. In
Drumochter Pass, an elevation of 1530 feet, potatoes can scarcely be
raised; and from 1000 to 1200 feet is the more common limit of the
cereal and the esculent. On this point a statement is made, which may
be useful to cultivators in the hill districts: it is, that 'the
common brake-fern (_Pteris aquilina_), distributed throughout Britain,
is found to be limited by a line running nearly level with the limit
of cultivation, and thus affords a test, when cultivation may be
absent, where nature does not deny it success. In one sheltered spot
in the woods of Loch-na-gar, it was observed at 1900 feet; and in
another part of the same woods, at 1700 feet; but on the exposed moors
it is very seldom seen beyond 1200 feet, unless in hollows, or on
declivities facing the sun.'
In accounting for the varieties of plants in Britain, it is assumed
that, during the glacial period, when the tops of our mountains were
mere islands in a great sea, under which lay the greater part of
modern Europe, they were then peopled by the arctic and alpine
species, which now inhabit them. Then came an upheaval; a vast tract
of land rose above the water, without any break, as at present between
England and the continent; and at this period 'there appears to have
been a migration of both plants and animals from east to west, the
descendants of which still constitute the great body of the flora and
fauna of the British lowlands.' Meantime, the elevation of the former
islands into mountain summits, placed them in a temperature suited to
the perpetuation of their vegetation. Then, to account for the
presence of a Spanish flora in the west of Ireland, a bold hypothesis,
started by Professor Edward Forbes, is put forward--'that the west of
Ireland was geologically united with the north of Spain;' admitting
which, there is no difficulty in supposing the plants to have
travelled along the intervening land, which has subsequently
disappeared, and that, owing to climatic changes, the hardier sort of
plants, such as saxifrages and heaths, have alone survived.