CHAP. X.] MEASUREMENT OF GEOLOGICAL TIME. 



229 



have ample material for that power to act upon, so as to keep 

 the organic world in a state of rapid change and development 

 proportioned to the comparatively rapid changes in the earth's 

 surface. 



We have now finished the series of preliminary stiidies of the 

 biological conditions and physical changes which have affected 

 the modification and dispersal of organisms, and have thus 

 brought about their actual distribution on the surface of the 

 earth. These studies will, it is believed, place us in a condition 

 to solve most of the problems presented by the distribution of 

 animals and plants, whenever the necessary facts, both as to their 

 distribution and their affinities, are sufficiently well known ; and 

 we now proceed to apply the principles we have established to 

 the interpretation of the phenomena presented by some of the 

 more important and best known of the islands of our globe, 

 limiting ourselves to these for reasons which have been already 

 sufficiently explained in our preface. 



