PRE FACE. 



The present volume is the result of four years' additional 

 thought and research on the lines laid down in my Geographical 

 Distribution of Animals, and may be considered as a popular 

 supplement to and completion of that work. 



It is, however, at the same time a complete work in itself ; 

 and, from the mode of treatment adopted, it will, I hope, be well 

 calculated to bring before the intelligent reader the wide scope 

 and varied interest of this branch of natural history. Although 

 some of the earlier chapters deal with the same questions as my 

 former volumes, they are here treated from a different point of 

 view ; and as the discussion of them is more elementary and at 

 the same time tolerably full, it is hoped that they will prove 

 both instructive and interesting. The plan of my larger work 

 required that genera only should be taken account of; in the 

 present volume I often discuss the distribution of species, and 

 this will help to render the work more intelligible to the 

 unscientific reader. 



The full statement of the scope and object of the present 

 essay given in the Introductory " chapter, together with the 

 "Summary" of the whole work and the general view of the 

 more important arguments given in the Conclusion," render it 

 unnecessary for me to offer any further remarks on these points. 

 I may, however, state generally that, so far as I am able to 



