Part 2

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.

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. 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.

“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.