IXTRODUCTORY 11 



been recognised ; and it is in order to direct the attention 

 of naturalists to this most promising field of research, that 

 I restrict myself in this volume to an elucidation of some 

 of the problems they jDresent to us. By far the larger 

 part of the islands of the globe are but portions of contin- 

 ents undergoing some of the various changes to which they 

 are ever subject ; and the correlative proposition, that every 

 portion of our continents has again and again passed 

 through insular conditions, has not been sufficiently con- 

 sidered, but is, I believe, the statement of a great and 

 most suo'o^estive truth, and one Avhich lies at the founda- 

 tion of all accurate conce23tion of the physical and organic 

 changes which have resulted in the jDresent state of the 

 earth. 



The indications now given of the scope and j^urjDose of 

 the present volume renders it evident that, before we can 

 proceed to the discussion of the remarkable phenomena 

 presented by insular faunas and floras, and the comi^lex 

 causes which have produced them, we must go through a 

 series of preliminary studies, adapted to give us a command 

 of the more important facts and principles on which the 

 solution of such problems depends. The succeeding 

 eight chapters will therefore be devoted to the explanation 

 of the mode of distribution, variation, modification, and 

 dispersal, of species and groups, illustrated by facts and 

 examples ; of the true nature of geological change as 

 affecting continents and islands ; of changes of climate, 

 their nature, causes, and effects ; of the duration of oreo- 

 logical time and the rate of organic development. 



