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 studies 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. 
