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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 dis

tinguish 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.


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