CHAP. ].] 
INTRODUCTORY. 
11 
accurate conception of the physical and organic changes which 
have resulted in the present state of the eaith. 
The indications now given of the scope and purpose 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 complex 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 geological time 
and the rate of organic development. 
