Chapter I: 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_