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TROPICAL NATURE,
AND OTHER ESSAYS.
[Illustration]
TROPICAL NATURE,
AND
OTHER ESSAYS.
BY
ALFRED R. WALLACE.
AUTHOR OF “THE MALAY ARCHIPELAGO,” “THE GEOGRAPHICAL DISTRIBUTION
OF ANIMALS,”“CONTRIBUTIONS TO THE THEORY OF NATURAL SELECTION,”
ETC., ETC.
London:
MACMILLAN AND CO.
1878.
[_The Right of Translation and Reproduction is Reserved._]
LONDON:
R. CLAY, SONS, AND TAYLOR,
BREAD STREET HILL.
THE TROPICS.
Land of the Sun! where joyous green-robed Spring
And leaf-crowned Summer deck the Earth for ever;
No Winter stern their sweet embrace to sever
And numb to silence every living thing,
But bird and insect ever on the wing,
Flitting ’mid forest glades and tangled bowers,
While the life-giving orb’s effulgent beams
Through all the circling year call forth the flowers.
Here graceful palms, here luscious fruits have birth;
The fragrant coffee, life-sustaining rice,
Sweet canes, and wondrous gums, and odorous spice;
While Flora’s choicest treasures crowd the teeming earth.
Beside each cot the golden Orange stands,
And broad-leaved Plantain, pride of Tropic lands.
ENGLAND.
Sweet changing Seasons! Winter cold and stern,
Fair Spring with budding leaf and opening flower,
And Summer when the sun’s creative power
Brings leafy groves and glades of feathery fern,
The glorious blossoms of sweet-scented May,
The flowery hedgerows and the fragrant hay,
And the wide landscape’s many-tinted sheen.
Then Autumn’s yellow woods and days serene;
And when we’ve gathered in the harvest’s treasure,
The long nights bring us round the blazing hearth,
The chosen haunt of every social pleasure.
Land of green fields and flowers! Thou givest birth
To shifting scenes of beauty, which outshine
Th’ unvarying splendours of the Tropic’s clime.
PREFACE.
The luxuriance and beauty of Tropical Nature is a well-worn
theme, and there is little new to say about it. The traveller and
the naturalist have combined to praise, and not unfrequently to
exaggerate the charms of tropical life--its heat and light, its
superb vegetable forms, its brilliant tints of flower and bird
and insect. Each strange and beautiful object has been described
in detail; and both the scenery and the natural phenomena of the
tropics have been depicted by master hands and with glowing colours.
But, so far as I am aware, no one has yet attempted to give a
general view of the phenomena which are _essentially_ tropical,
or to determine the causes and conditions of those phenomena. The
local has not been separated from the general, the accidental from
the essential; and, as a natural result, many erroneous ideas have
become current as to what are really the characteristics of the
tropical as distinguished from the temperate zones.
In the present volume I have attempted to supply this want; and for
my materials have drawn chiefly on my own twelve years’ experience
of the eastern and western tropics of the equatorial zone, where
the characteristic phenomena of tropical life are fully manifested.
So many of the most remarkable forms of life are now restricted to
the tropics, and the relations of these to extinct types which once
inhabited the temperate zones open up so many interesting questions
as to the past history of the earth, that the present inquiry may be
considered a necessary preliminary to a study of the problem--how
to determine the climates of geologic periods from the character
of their organic remains. This part of the subject is however both
complex and difficult, and I have only attempted to indicate what
seem to me the special physical conditions to which the existing
peculiarities of tropical life are mainly due.
The three opening chapters treat the subject under the headings of
climate, vegetation, and animal life. The conditions and causes
of the equatorial climate are discussed in some detail, and the
somewhat complex principles on which it depends are popularly
explained. In the chapters on plant and animal life, the general
aspects and relations of their several component elements have been
dwelt upon; all botanical and zoological details and nomenclature
being excluded, except so far as was absolutely necessary to give
precision to the descriptions and to enable us to deduce from them
some conclusions of importance.
The remaining chapters have all a more or less direct connection
with the leading subject. The family of humming-birds is taken as
an illustration of the luxuriant development of allied forms in the
tropics, and as showing the special mode in which natural selection
has acted to bring about considerable changes in a limited period.
The discussion on the nature and origin of the colours of animals
and plants, is intended to show how far and in what way these are
dependent on the climate and physical conditions of the tropics. The
chapter entitled “By-paths in the Domain of Biology” contains an
account of certain curious relations of colour to locality, which
are almost exclusively manifested within the tropical zones; while
the essay on “Distribution of Animals and Geographical Changes,”
elucidates the relations of the several continents in past time,
and the probable origin of many of the groups now characteristic
of tropical or of temperate regions.
While discussing the general laws and phenomena of colour in the
organic world, and its special developments among certain groups
of animals, I have been led to a theory of the diverse colours of
the sexes and of the special ornaments and brilliant hues which
distinguish certain male birds and insects, which is directly
opposed to the view held by Mr. Darwin and so well explained and
illustrated in his great work on “The Descent of Man and on Selection
in Relation to Sex.” Being strongly impressed with the importance
and fundamental truth of this theory, I published my first sketch of
the subject in _Macmillan’s Magazine_ in order that it might have
the benefit of criticism before making it public in a more permanent
form. Taking advantage of some suggestions from Mr. Darwin and from
a few other correspondents, I have made considerable additions to
the original essay and have rearranged, and I trust strengthened
the argument, which I now hope may attract the attention of all who
are interested in the subject. I may be allowed here to remark,
that my theory cannot be properly understood without reading the
whole chapter on “The Colours of Animals;” because the view set
forth and illustrated in the first part of that chapter--that colour
in nature is normal, and that its presence hardly requires to be
accounted for so much as its absence--is an essential part of the
theory.
CROYDON, _April, 1878_.
CONTENTS.
I. The Climate and Physical Aspects of the Equatorial Zone.
The three Climatal Zones of the Earth--Temperature of the
Equatorial Zone--Causes of the Uniform High Temperature near
the Equator--Influence of the Heat of the Soil--Influence of
the Aqueous Vapour of the Atmosphere--Influence of Winds on
the Temperature of the Equator--Heat due to the Condensation
of Atmospheric Vapour--General Features of the Equatorial
Climate--Uniformity of the Equatorial Climate in all Parts of the
Globe--Effects of Vegetation on Climate--Short Twilight of the
Equatorial Zone--The Aspect of the Equatorial Heavens--Intensity
of Meteorological Phenomena at the Equator--Concluding Remarks
_pages 1-26_
II. Equatorial Vegetation.
The Equatorial Forest-belt and its Causes--General Features of the
Equatorial Forests--Low-growth Forest-trees--Flowery Trunks
and their Probable Cause--Uses of Equatorial Forest-trees--The
Climbing Plants of the Equatorial Forests--Palms--Uses of
Palm-trees and their Products--Ferns--Ginger-worts and Wild
Bananas--Arums--Screw-Pines--Orchids--Bamboos--Uses of the
Bamboo--Mangroves--Sensitive-plants--Comparative Scarcity of
Flowers--Concluding Remarks on Tropical Vegetation _pages 27-68_
III. Animal Life in the Tropical Forests.
Difficulties of the Subject--General Aspect of the Animal Life of
Equatorial Forests--Diurnal Lepidoptera or Butterflies--Peculiar
Habits of Tropical Butterflies--Ants, Wasps, and
Bees--Ants--Special Relations between Ants and Vegetation--Wasps
and Bees--Orthoptera and other Insects--Beetles--Wingless
Insects--General Observations on Tropical
Insects--Birds--Parrots--Pigeons--Picariæ--Cuckoos--Trogons,
Barbets, Toucans and Hornbills--Passeres--Reptiles
and Amphibia--Lizards--Snakes--Frogs and
Toads--Mammalia--Monkeys--Bats--Summary of the Aspects of Animal
Life in the Tropics _pages 69-123_
IV. Humming-Birds: as Illustrating the Luxuriance of Tropical Nature.
Structure--Colours and Ornaments--Descriptive Names--The
Motions and Habits of Humming-Birds--Display of Ornaments
by the Male--Food--Geographical Distribution and
Variation--Humming-Birds of Juan Fernandez as illustrating
Variation and Natural Selection--The Relations and Affinities of
Humming-Birds--How to Determine Doubtful Affinities--Resemblances
of Swifts and Humming-Birds--Differences between Sun-Birds and
Humming-Birds _pages 124-157_
V. The Colours of Animals and Sexual Selection.
General Phenomena of Colour--Theory of Heat and Light as
producing Colour--Changes of Colour in Animals produced by
Coloured Light--Classification of Organic Colours--Protective
Colours--Warning Colours--Sexual Colours--Typical Colours--The
Nature of Colour--How Animal Colours are Produced--Colour
a Normal Product of Organization--Theory of Protective
Colours--Theory of Warning Colours--Imitative Warning
Colours--The Theory of Mimicry--Theory of Sexual Colours--Colour
as a Means of Recognition--Colour proportionate to Integumentary
Development--Selection by Females not a Cause of Colour--Probable
Use of the Horns of Beetles--Cause of the greater Brilliancy of
some Female Insects--Origin of the Ornamental Plumage of Male
Birds--Theory of the Display of Ornaments by Males--Natural
Selection as neutralizing Sexual Selection--Greater Brilliancy
of some Female Birds--Colour-development as illustrated
by Humming-Birds--Theory of Typical Colours--Local Causes
of Colour-development--Summary on Colour-development in
Animals--Concluding Remarks on Causes of Bright Colour in the
Tropics _pages 158-220_
VI. The Colours of Plants and the Origin of the Colour-Sense.
Source of Colouring-matter in Plants--Protective Coloration and
Mimicry in Plants--Attractive Colours of Fruits--Protective
Colours of Fruits--Seeds how Protected--Attractive Colours of
Flowers--Attractive Odours in Flowers--Attractive Grouping
in Flowers--Why Alpine Flowers are so Beautiful--Why Allied
Species of Flowers differ in Size and Beauty--Absence of
Colours in Wind-fertilized Flowers--The same Theory of Colour
applicable to Animals and Plants--Relation of the Colours of
Flowers and their Geographical Distribution--Recent Views as
to the Direct Action of Light on the Colours of Flowers and
Fruits--Concluding Remarks on the Importance of Colour in the
Organic World--The Origin of the Colour-sense.--Supposed Increase
of Colour-perception within the Historical Period--Concluding
Remarks on the Colour-sense _pages 221-248_
VII. BY-PATHS IN THE DOMAIN OF BIOLOGY.
BEING AN ADDRESS DELIVERED TO THE BIOLOGICAL SECTION OF THE BRITISH
ASSOCIATION (GLASGOW, SEPTEMBER 6TH, 1876) AS PRESIDENT OF THE
SECTION.
Introductory Remarks--ON SOME RELATIONS OF LIVING THINGS TO
THEIR ENVIRONMENT.--The Influence of Locality on Colour in
Butterflies and Birds--Sense-perception influenced by Colour of
the Integuments--Relations of Insular Plants and Insects--RISE
AND PROGRESS OF MODERN VIEWS AS TO THE ANTIQUITY AND ORIGIN
OF MAN--Indications of Man’s Extreme Antiquity--Antiquity of
Intellectual Man--Sculptures on Easter-Island--North American
Earthworks--The Great Pyramid--Conclusion _pages 249-303_
VIII. The Distribution of Animals as Indicating Geographical Changes.
Old Opinions on Continental Changes--Theory of Oceanic
Islands--Present and Past Distribution of Land and
Sea--Zoological Regions--The Palæarctic Region--The Ethiopian
Region--The Oriental Region--Past changes of the Great Eastern
Continent--Regions of the New World--Past History of the American
Continents--The Australian Region--Summary and Conclusion
_pages 304-347_
TROPICAL NATURE,
AND OTHER ESSAYS.
I.
THE CLIMATE AND PHYSICAL ASPECTS OF THE EQUATORIAL ZONE.
The three Climatal Zones of the Earth--Temperature of the
Equatorial Zone--Causes of the Uniform High Temperature near
the Equator--Influence of the Heat of the Soil--Influence of
the Aqueous Vapour of the Atmosphere--Influence of Winds on
the Temperature of the Equator--Heat due to the Condensation
of Atmospheric Vapour--General features of the Equatorial
Climate--Uniformity of the Equatorial Climate in all parts of the
globe--Effects of Vegetation on Climate--Short Twilight of the
Equatorial Zone--The aspect of the Equatorial Heavens--Intensity
of meteorological phenomena at the Equator--Concluding Remarks.
It is difficult for an inhabitant of our temperate land to realize
either the sudden and violent contrasts of the arctic seasons or the
wonderful uniformity of the equatorial climate. The lengthening or
the shortening days, the ever-changing tints of spring, summer, and
autumn, succeeded by the leafless boughs of winter, are constantly
recurring phenomena which represent to us the established course
of nature. At the equator none of these changes occur; there is
a perpetual equinox and a perpetual summer, and were it not for
variations in the quantity of rain, in the direction and strength
of the winds, and in the amount of sunshine, accompanied by
corresponding slight changes in the development of vegetable and
animal life, the monotony of nature would be extreme.
In the present chapter it is proposed to describe the chief
peculiarities which distinguish the equatorial from the temperate
climate, and to explain the causes of the difference between
them,--causes which are by no means of so simple a nature as are
usually imagined.
The three great divisions of the earth--the tropical, the
temperate, and the frigid zones, may be briefly defined as the
regions of uniform, of variable, and of extreme physical conditions
respectively. They are primarily determined by the circumstance
of the earth’s axis not being perpendicular to the plane in which
it moves round the sun; whence it follows that during one half
of its revolution the north pole, and during the other half the
south pole, is turned at a considerable angle towards the source
of light and heat. This inclination of the axis on which the earth
rotates is usually defined by the inclination of the equator to
the plane of the orbit, termed the obliquity of the ecliptic. The
amount of this obliquity is 23½ degrees, and this measures the
extent on each side of the equator of what are called the tropics,
because within these limits the sun becomes vertical at noon twice
a year, and at the extreme limit once a year, while beyond this
distance it is never vertical. It will be evident, however, from
the nature of the case, that the two lines which mark the limits of
the geographical “tropics” will not define any abrupt change of
climate or physical conditions, such as characterise the tropical
and temperate zones in their full development. There will be a
gradual transition from one to the other, and in order to study
them separately and contrast their special features we must only
take into account the portion of each in which these are most fully
exhibited. For the temperate zone we may take all countries situated
between 35° and 60° of latitude, which in Europe will include
every place between Christiania and Algiers, the districts further
south forming a transitional belt in which temperate and tropical
features are combined. In order to study the special features of
tropical nature, on the other hand, it will be advisable to confine
our attention mainly to that portion of the globe which extends for
about twelve degrees on each side of the equator, in which all the
chief tropical phenomena dependent on astronomical causes are most
fully manifested, and which we may distinguish as the “equatorial
zone.” In the debateable ground between these two well contrasted
belts local causes have a preponderating influence; and it would not
be difficult to point out localities within the temperate zone of
our maps, which exhibit all the chief characteristics of tropical
nature to a greater degree than other localities which are, as
regards geographical position, tropical.
_Temperature of the Equatorial Zone._--The most characteristic,
as it is the most important feature in the physical conditions
of the great equatorial zone is the wonderful uniformity of its
temperature, alike throughout the changes of day and night, and from
one part of the year to another. As a general rule, the greatest
heat of the day does not exceed 90° or 91° Fahr., while it seldom
falls during the night below 74° Fahr. It has been found by hourly
observations carried on for three years at the meteorological
observatory established by the Dutch government at Batavia, that
the extreme range of temperature in that period was only 27° Fahr.,
the maximum being 95° and the minimum 68°. But this is, of course,
very much beyond the usual daily range of the thermometer, which
is, on the average, only a little more than 11° Fahr.; being 12·6°
in September when it is greatest, and only 8·1° in January, when
it is least.
Batavia, being situated between six and seven degrees south of the
equator, may be taken as affording a fair example of the climate
of the equatorial zone; though, being in an island, it is somewhat
less extreme than many continental localities. Observations made
at Para, which is continental and close to the equator, agree
however very closely with those at Batavia; but at the latter place
all the observations were made with extreme care and with the
best instruments, and are therefore preferred as being thoroughly
trustworthy.[1] The accompanying diagram, showing by curves the
monthly means of the highest and lowest daily temperatures at Batavia
and London, is very instructive; more especially when we consider
that the maximum of temperature is by no means remarkably different
in the two places, 90° Fahr, being sometimes reached with us and
not being often very much exceeded at Batavia.
[1] “Observations Made at the Magnetical and Meteorological
Observatory at Batavia. Published by order of the Government of
Netherlands India. Vol. I. Meteorological, from Jan. 1866 to Dec.
1868; and Magnetical, from July 1867 to June 1870. By Dr. P. A.
Bergsma. Batavia, 1871.” This fine work is entirely in English.
[Illustration: _Monthly Mean Temperature at Batavia & London._]
_Causes of the Uniform High Temperature near the Equator._--It is
popularly supposed that the uniform high temperature of the tropics
is sufficiently explained by the greater altitude, and therefore
greater heating-power, of the midday sun; but a little consideration
will show that this alone by no means accounts for the phenomenon.
The island of Java is situated in from six and a half to eight and
a half degrees of south latitude, and in the month of June the
sun’s altitude at noon will not be more than from 58° to 60°. In
the same month at London, which is fifty-two and a half degrees
of north latitude, the sun’s noonday altitude is 62°. But besides
this difference of altitude in favour of London there is a still
more important difference; for in Java the day is only about eleven
and a half hours long in the month of June, while at London it is
sixteen hours long, so that the total amount of sun-heat received by
the earth must be then very much greater at London than at Batavia.
Yet at the former place the mean temperature of the day and night
is under 60° Fahr., while in the latter place it is 80° Fahr., the
daily maximum being on the average in the one case about 68° and
in the other about 89°.
Neither does the temperature at the same place depend upon the
height of the sun at noon; for at Batavia it is nearly vertical
during October and February, but these are far from being the
hottest months, which are May, June, and September; while December,
January, and February are the coldest months, although then the sun
attains nearly its greatest altitude. It is evident, therefore,
that a difference of 30° in the altitude of the sun at noon has no
apparent influence in raising the temperature of a place near the
equator, and we must therefore conclude that other agencies are at
work which often completely neutralise the effect which increased
altitude must undoubtedly exert.
There is another important difference between the temperate and
tropical zones, in the direct heating effect of the sun’s rays
independently of altitude. In England the noonday sun in the month of
June rarely inconveniences us or produces any burning of the skin;
while in the tropics, at almost any hour of the day, and when the
sun has an elevation of only 40° or 50°, exposure to it for a few
minutes will scorch a European so that the skin turns red, becomes
painful, and often blisters or peels off. Almost every visitor to the
tropics suffers from incautious exposure of the neck, the leg, or
some other part of the body to the sun’s rays, which there possess
a power as new, as it is at first sight inexplicable, for it is
not accompanied by any extraordinary increase in the temperature
of the air.
These very different effects, produced by the same amount of sun-heat
poured upon the earth in different latitudes is due to a combination
of causes. The most important of these are, probably,--the constant
high temperature of the soil and of the surface-waters of the
ocean,--the great amount of aqueous vapour in the atmosphere,--the
great extent of the intertropical regions which cause the winds that
reach the equatorial zone to be always warm,--and the latent heat
given out during the formation of rain and dew. We will briefly
consider the manner in which each of these causes contributes to
the degree and the uniformity of the equatorial temperature.
_Influence of the Heat of the Soil._--It is well known that at
a very moderate depth the soil maintains a uniform temperature
during the twenty-four hours; while at a greater depth even the
annual inequalities disappear, and a uniform temperature, which is
almost exactly the mean temperature of the locality, is constantly
maintained throughout the year. The depth at which this uniform
temperature is reached is greater as the annual range of temperature
is greater, so that it is least near the equator, and greatest in
localities near the arctic circle where the greatest difference
between summer and winter temperature prevails. In the vicinity of
the equator, where the annual range of the thermometer is so small
as we have seen that it is at Batavia, the mean temperature of about
80° Fahr. is reached at a depth of four or five feet. The surplus
heat received during the day is therefore conducted downwards very
slowly, the surface soil becomes greatly superheated, and a large
portion of this heat is given out at night and thus keeps up the
high temperature of the air when the sun has ceased to warm the
earth. In the temperate zones, on the other hand, the stratum of
uniform earth-temperature lies very deep. At Geneva it is not less
than from thirty to forty feet, and with us it is probably fifty or
sixty feet, and the temperature found there is nearly forty degrees
lower than at the equator. This great body of cool earth absorbs a
large portion of the surface heat during the summer, and conducts
it downwards with comparative rapidity, and it is only late in the
year (in July and August) when the upper layers of the soil have
accumulated a surplus store of solar heat that a sufficient quantity
is radiated at night to keep up a high temperature in the absence
of the sun. At the equator, on the other hand, this radiation is
always going on, and earth-heat is one of the most important of
the agencies which tend to equalise the equatorial climate.
_Influence of the Aqueous Vapour of the Atmosphere._--The aqueous
vapour which is always present in considerable quantities in the
atmosphere, exhibits a singular and very important relation to
solar and terrestrial heat. The rays of the sun pass through it
unobstructed to the earth; but the warmth given off by the heated
earth is very largely absorbed by it, thus raising the temperature
of the air; and as it is the lower strata of air which contain most
vapour these act as a blanket to the earth, preventing it from losing
heat at night by radiation into space. During a large part of the
year the air in the equatorial zone is nearly saturated with vapour,
so that, notwithstanding the heat, salt and sugar become liquid,
and all articles of iron get thickly coated with rust. Complete
saturation being represented by 100, the daily average of greatest
humidity at Batavia reaches 96 in January and 92 in December. In
January, which is the dampest month, the range of humidity is small
(77 to 96), and at this time the range of temperature is also
least; while in September, with a greater daily range of humidity
(62 to 92) the range of temperature is the greatest, and the lowest
temperatures are recorded in this and the preceding month. It is a
curious fact, that in many parts of England the degree of humidity
as measured by the comparative saturation of the air, is as great
as that of Batavia or even greater. A register kept at Clifton
during the years 1853-1862 shows a mean humidity in January of 90,
while the highest monthly mean for the four years at Batavia was
88; and while the lowest of the monthly means at Clifton was 79·1,
the lowest at Batavia was 78·9. These figures however represent
an immense difference in the _quantity_ of vapour in every cubic
foot of air. In January at Clifton, with a temperature of 35° to
40° Fahr., there would be only about 4 to 4½ grains of vapour per
cubic foot of air, while at Batavia, with a temperature from 80°
to 90° Fahr., there would be about 20 grains in the same quantity
of air. The most important fact however is, that the capacity of
air for holding vapour in suspension increases more rapidly than
temperature increases, so that a fall of ten degrees at 50° Fahr.
will lead to the condensation of about 1½ grains of vapour, while
a similar fall at 90° Fahr. will set free 6½ grains. We can thus
understand how it is that the very moderate fall of the thermometer
during a tropical night causes heavier dews and a greater amount
of sensible moisture than are ever experienced during much greater
variations of temperature in the temperate zone. It is this large
quantity of vapour in the equatorial atmosphere that keeps up a
genial warmth throughout the night by preventing the radiation into
space of the heat absorbed by the surface soil during the day. That
this is really the case is strikingly proved by what occurs in the
plains of Northern India, where the daily maximum of heat is far
beyond anything experienced near the equator, yet, owing to the
extreme dryness of the atmosphere, the clear nights are very cold,
radiation being sometimes so rapid that water placed in shallow
pans becomes frozen over.
As the heated earth, and everything upon its surface, does not cool
so fast when surrounded by moist as by dry air, it follows, that
even if the quantity and intensity of the solar rays falling upon
two given portions of the earth’s surface are exactly equal, yet the
sensible and effective heat produced in the two localities may be
very different according as the atmosphere contains much or little
vapour. In the one case the heat is absorbed more rapidly than it
can escape by radiation; in the other case it radiates away into
space, and is lost, more rapidly than it is being absorbed. In both
cases an equilibrium will be arrived at, but in the one case the
resulting mean temperature will be much higher than in the other.
_Influence of Winds on the Temperature of the Equator._--The distance
from the northern to the southern tropics being considerably more
than three thousand miles, and the area of the intertropical
zone more than one-third the whole area of the globe, it becomes
hardly possible for any currents of air to reach the equatorial
belt without being previously warmed by contact with the earth or
ocean, or by mixture with the heated surface-air which is found in
all intertropical and subtropical lands. This warming of the air
is rendered more certain and more effective by the circumstance,
that all currents of air coming from the north or south have their
direction changed owing to the increasing rapidity of the earth’s
rotational velocity, so that they reach the equator as easterly
winds, and thus pass obliquely over a great extent of the heated
surface of the globe. The causes that produce the westerly monsoons
act in a similar manner, so that on the equator direct north or
south winds, except as local land and sea breezes, are almost
unknown. The Batavia observations show, that for ten months in
the year the average direction of the wind varies only between 5°
and 30° from due east or west, and these are also the strongest
winds. In the two months--March and October--when the winds are
northerly, they are very light, and are probably in great part
local sea-breezes, which, from the position of Batavia, must come
from the north. As a rule, therefore, every current of air at or
near the equator has passed obliquely over an immense extent of
tropical surface and is thus necessarily a warm wind.
In the north temperate zone, on the other hand, the winds are always
cool, and often of very low temperature even in the height of
summer, due probably to their coming from colder northern regions as
easterly winds, or from the upper parts of the atmosphere as westerly
winds; and this constant supply of cool air, combined with quick
radiation through a dryer atmosphere, carries off the solar heat
so rapidly that an equilibrium is only reached at a comparatively
low temperature. In the equatorial zone, on the contrary, the heat
accumulates, on account of the absence of any medium of sufficiently
low temperature to carry it off rapidly, and it thus soon reaches
a point high enough to produce those scorching effects which are
so puzzling when the altitude of the sun or the indications of
the thermometer are alone considered. Whenever, as is sometimes
the case, exceptional cold occurs near the equator, it can almost
always be traced to the influence of currents of air of unusually
low temperature. Thus in July near the Aru islands, the writer
experienced a strong south-east wind which almost neutralised the
usual effects of tropical heat although the weather was bright and
sunny. But the wind, coming direct from the southern ocean during
its winter without acquiring heat by passing over land, was of an
unusually low temperature. Again, Mr. Bates informs us that in the
Upper Amazon in the month of May there is a regularly recurring
south wind which produces a remarkable lowering of the usual
equatorial temperature. But owing to the increased velocity of the
earth’s surface at the equator a south wind there must have been
a south-west wind at its origin, and this would bring it directly
from the high chain of the Peruvian Andes during the winter of the
southern hemisphere. It is therefore probably a cold mountain wind,
and blowing as it does over a continuous forest it has been unable
to acquire the usual tropical warmth.
The cause of the striking contrast between the climates of equatorial
and temperate lands at times when both are receiving an approximately
equal amount of solar heat may perhaps be made clearer by an
illustration. Let us suppose there to be two reservoirs of water,
each supplied by a pipe which pours into it a thousand gallons a
day, but which runs only during the daytime, being cut off at night.
The reservoirs are both leaky, but while the one loses at the rate
of nine hundred gallons in the twenty-four hours the other loses
at the rate of eleven hundred gallons in the same time, supposing
that both are kept exactly half full and thus subjected to the same
uniform water-pressure. If now both are left to be supplied by the
above-mentioned pipes the result will be, that in the one which
loses by leakage less than it receives the water will rise day by
day, till the increased pressure causes the leakage to increase so
as exactly to balance the supply; while in the other the water will
sink till the decreasing pressure causes the leakage to decrease
so as to balance the supply, when both will remain stationary, the
one at a high the other at a low average level, each rising during
the day and sinking again at night. Just the same thing occurs
with that great heat-reservoir the earth, whose actual temperature
at any spot will depend, not alone upon the quantity of heat it
receives, but on the balance between its constantly varying waste
and supply. We can thus understand how it is that, although in the
months of June and July Scotland in latitude 57° north receives as
much sun-heat as Angola or Timor in latitude 10° south, and for
a much greater number of hours daily, yet in the latter the mean
temperature will be about 80° Fahr., with a daily maximum of 90°
to 95°, while in the former the mean will be about 60° Fahr. with
a daily maximum of 70° or 75°; and, while in Scotland exposure to
the full noonday sun produces no unpleasant heat-sensations, a
similar exposure in Timor at any time between 9 A.M. and 3 P.M.
would blister the skin in a few minutes almost as effectually as
the application of scalding water.
_Heat Due to the Condensation of Atmospheric Vapour._--Another cause
which tends to keep up a uniform high temperature in the equatorial,
as compared with the variable temperatures of the extra-tropical
zones, is the large amount of heat liberated during the condensation
of the aqueous vapour of the atmosphere in the form of rain and dew.
Owing to the frequent near approach of the equatorial atmosphere
to the saturation point, and the great weight of vapour its high
temperature enables it to hold in suspension, a very slight fall
of the thermometer is accompanied by the condensation of a large
absolute quantity of atmospheric vapour, so that copious dews and
heavy showers of rain are produced at comparatively high temperatures
and low altitudes. The drops of rain rapidly increase in size while
falling through the saturated atmosphere; and during this process
as well as by the formation of dew, the heat which retained the
water in the gaseous form, and was insensible while doing so, is
liberated, and thus helps to keep up the high temperature of the air.
This production of heat is almost always going on. In fine weather
the nights are always dewy, and the diagram on the preceding page
showing the mean monthly rainfall at Batavia and Greenwich proves
that this source of increased temperature is present during every
month in the year, since the lowest monthly fall at the former
place is almost equal to the highest monthly fall at the latter.
[Illustration: _Monthly Rainfall at London and Batavia._]
It may perhaps be objected, that evaporation must absorb as much heat
as is afterwards liberated by condensation, and this is true; but as
evaporation and condensation occur usually at different times and
in different places, the equalising effect is still very important.
Evaporation occurs chiefly during the hottest sunshine, when it
tends to moderate the extreme heat, while condensation takes place
chiefly at night in the form of dew and rain, when the liberated
heat helps to make up for the loss of the direct rays of the sun.
Again, the most copious condensation both of dew and rain is greatly
influenced by vegetation and especially by forests, and also by the
presence of hills and mountains, and is therefore greater on land
than on the ocean; while evaporation is much greater on the ocean,
both on account of the less amount of cloudy weather and because
the air is more constantly in motion. This is particularly the case
throughout that large portion of the tropical and subtropical
zones where the trade-winds constantly blow, as the evaporation
must there be enormous while the quantity of rain is very small.
It follows, then, that on the equatorial land-surface there will
be a considerable balance of condensation over evaporation which
must tend to the general raising of the temperature, and, owing to
the condensation being principally at night, not less powerfully
to its equalisation.
_General Features of the Equatorial Climate._--The various causes now
enumerated are sufficient to enable us to understand how the great
characteristic features of the climate of the equatorial zone are
brought about; how it is that so high a temperature is maintained
during the absence of the sun at night, and why so little effect
is produced by the sun’s varying altitude during its passage from
the northern to the southern tropic. In this favoured zone the heat
is never oppressive, as it so often becomes on the borders of the
tropics; and the large absolute amount of moisture always present
in the air, is almost as congenial to the health of man as it is
favourable to the growth and development of vegetation.[2] Again,
the lowering of the temperature at night is so regular and yet so
strictly limited in amount, that, although never cold enough to be
unpleasant, the nights are never so oppressively hot as to prevent
sleep. During the wettest months of the year, it is rare to have
many days in succession without some hours of sunshine, while
even in the driest months there are occasional showers to cool
and refresh the overheated earth. As a result of this condition
of the earth and atmosphere, there is no check to vegetation, and
little if any demarcation of the seasons. Plants are all evergreen;
flowers and fruits, although more abundant at certain seasons, are
never altogether absent; while many annual food-plants as well as
some fruit-trees produce two crops a year. In other cases, more
than one complete year is required to mature the large and massive
fruits, so that it is not uncommon for fruit to be ripe at the same
time that the tree is covered with flowers, in preparation for the
succeeding crop. This is the case with the Brazil nut tree, in the
forests of the Amazon, and with many other tropical as with a few
temperate fruits.
[2] Where the inhabitants adapt their mode of life to the
peculiarities of the climate, as is the case with the Dutch in
the Malay Archipelago, they enjoy as robust health as in Europe,
both in the case of persons born in Europe and of those who for
generations have lived under a vertical sun.
_Uniformity of the Equatorial Climate in all Parts of the
Globe._--The description of the climatal phenomena of the equatorial
zone here given, has been in great part drawn from long personal
experience in South America and in the Malay Archipelago. Over
a large portion of these countries the same general features
prevail, only modified by varying local conditions. Whether we
are at Singapore or Batavia; in the Moluccas, or New Guinea; at
Para, at the sources of the Rio Negro, or on the Upper Amazon, the
equatorial climate is essentially the same, and we have no reason
to believe that it materially differs in Guinea or the Congo. In
certain localities, however, a more contrasted wet and dry season
prevails, with a somewhat greater range of the thermometer. This is
generally associated with a sandy soil, and a less dense forest,
or with an open and more cultivated country. The open sandy
country with scattered trees and shrubs or occasional thickets,
which is found at Santarem and Monte-Alegre on the lower Amazon,
are examples, as well as the open cultivated plains of Southern
Celebes; but in both cases the forest country in adjacent districts
has a moister and more uniform climate, so that it seems probable
that the nature of the soil or the artificial clearing away of the
forests, are important agents in producing the departure from the
typical equatorial climate observed in such districts. The almost
rainless district of Ceara on the North-East coast of Brazil and
only a few degrees south of the equator, is a striking example
of the need of vegetation to react on the rainfall. We have here
no apparent cause but the sandy soil and bare hills, which when
heated by the equatorial sun produce ascending currents of warm
air and thus prevent the condensation of the atmospheric vapour,
to account for such an anomaly; and there is probably no district
where judicious planting would produce such striking and beneficial
effects. In Central India the scanty and intermittent rainfall,
with its fearful accompaniment of famine, is no doubt in great part
due to the absence of a sufficient proportion of forest-covering
to the earth’s surface; and it is to a systematic planting of all
the hill tops, elevated ridges, and higher slopes that we can alone
look for a radical cure of the evil. This would almost certainly
induce an increased rainfall; but even more important and more
certain, is the action of forests in checking evaporation from the
soil and causing perennial springs to flow, which may be collected
in vast storage tanks and will serve to fertilise a great extent
of country; whereas tanks without regular rainfall or permanent
springs to supply them are worthless. In the colder parts of the
temperate zones, the absence of forests is not so much felt, because
the hills and uplands are naturally clothed with a thick coating
of turf which absorbs moisture and does not become overheated by
the sun’s rays, and the rains are seldom violent enough to strip
this protective covering from the surface. In tropical and even
in south-temperate countries, on the other hand, the rains are
periodical and often of excessive violence for a short period;
and when the forests are cleared away the torrents of rain soon
strip off the vegetable soil, and thus destroy in a few years the
fertility which has been the growth of many centuries. The bare
subsoil becoming heated by the sun, every particle of moisture
which does not flow off is evaporated, and this again reacts on the
climate, producing long-continued droughts only relieved by sudden
and violent storms, which add to the destruction and render all
attempts at cultivation unavailing. Wide tracts of fertile land in
the south of Europe have been devastated in this manner, and have
become absolutely uninhabitable. Knowingly to produce such disastrous
results would be a far more serious offence than any destruction of
property which human labour has produced and can replace; yet we
ignorantly allow such extensive clearings for coffee cultivation
in India and Ceylon, as to cause the destruction of much fertile
soil which generations cannot replace, and which will surely, if
not checked in time, lead to the deterioration of the climate and
the permanent impoverishment of the country.[3]
[3] For a terrible picture of the irreparable devastation caused
by the reckless clearing of forests see the third chapter of Mr.
Marsh’s work _The Earth as Modified by Human Action_.
_Short Twilight of the Equatorial Zone._--One of the phenomena
which markedly distinguish the equatorial from the temperate and
polar zones, is the shortness of the twilight and consequent
rapid transition from day to night and from night to day. As this
depends only on the fact of the sun descending vertically instead
of obliquely below the horizon, the difference is most marked when
we compare our midsummer twilight with that of the tropics. Even
with us the duration of twilight is very much shorter at the time
of the equinoxes, and it is probably not much more than a third
shorter than this at the equator. Travellers usually exaggerate the
shortness of the tropical twilight, it being sometimes said that if
we turn a page of the book we are reading when the sun disappears,
by the time we turn over the next page it will be too dark to see to
read. With an average book and an average reader this is certainly
not true, and it will be well to describe as correctly as we can
what really happens.
In fine weather the air appears to be somewhat more transparent near
the equator than with us, and the intensity of sunlight is usually
very great up to the moment when the solar orb touches the horizon.
As soon as it has disappeared the apparent gloom is proportionally
great, but this hardly increases perceptibly during the first ten
minutes. During the next ten minutes however it becomes rapidly
darker, and at the end of about twenty-five minutes from sunset
the complete darkness of night is almost reached. In the morning
the changes are perhaps even more striking. Up to about a quarter
past five o’clock the darkness is complete; but about that time a
few cries of birds begin to break the silence of night, perhaps
indicating that signs of dawn are perceptible in the eastern
horizon. A little later the melancholy voices of the goatsuckers
are heard, varied croakings of frogs, the plaintive whistle of
mountain thrushes, and strange cries of birds or mammals peculiar
to each locality. About half-past five the first glimmer of light
becomes perceptible; it slowly becomes lighter, and then increases so
rapidly that at about a quarter to six it seems full daylight. For
the next quarter of an hour this changes very little in character;
when, suddenly, the sun’s rim appears above the horizon, decking the
dew-laden foliage with glittering gems, sending gleams of golden
light far into the woods, and waking up all nature to life and
activity. Birds chirp and flutter about, parrots scream, monkeys
chatter, bees hum among the flowers, and gorgeous butterflies
flutter lazily along or sit with fully expanded wings exposed to
the warm and invigorating rays. The first hour of morning in the
equatorial regions possesses a charm and a beauty that can never
be forgotten. All nature seems refreshed and strengthened by the
coolness and moisture of the past night; new leaves and buds unfold
almost before the eye, and fresh shoots may often be observed to
have grown many inches since the preceding day. The temperature is
the most delicious conceivable. The slight chill of early dawn,
which was itself agreeable, is succeeded by an invigorating warmth;
and the intense sunshine lights up the glorious vegetation of the
tropics, and realises all that the magic art of the painter or
the glowing words of the poet, have pictured as their ideals of
terrestrial beauty.