PREFACE. ix 



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 develop- 

 ments 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- 



