PEEFACE 



In the present volume I have attempted to summarise and 

 complete my half-century of thought and work on the Dar- 

 winian theory of evolution. In several directions, I have 

 extended the scope and application of the theory, and have 

 shown that it is capable of explaining many of the phenomena 

 of living things hitherto thought to be beyond its range. 



Among these are the detailed distribution of plants and 

 animals, which I have discussed at some length. It occupies 

 about one-fourth of the volume (Chapters II. to VI.)? and 

 brings out certain facts and conclusions which I believe mil 

 be of interest to all plant-lovers, and also be not without a 

 certain value to botanists. 



'Next in importance are three chapters (X., XL, and XII.) 

 devoted to a general review of the Geological Eecord and a 

 discussion of the various problems arising out of it. Some 

 of the conclusions to which this examination leads us are, I 

 believe, both important and of much general interest. 



In Chapter VIII. I have endeavoured to show natural selec- 

 tion actually at work in the continually perfecting that won- 

 derful co-adaptation of the most diverse forms of life which 

 pervades all nature. Some little-known aspects of bird-migra- 

 tion are here discussed, and proof is given of the enormous 

 importance of mosquitoes for the very existence of considerable 

 proportion of our birds, including most of our most favoured 

 pets and songsters. This chapter will, I think, have a special 

 interest for every bird-lover. 



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