PREFACE. 
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 
