CHAP. XXIV SUMMARY AND CONCLUSION 533 



of which are shown to exhibit certain well-marked biological 

 features. 



Having thus shown that the work is a connected whole, 

 founded on the principle of tracing out the more recondite 

 causes of the distribution of organisms, we will briefly 

 indicate the scope and object of the several chapters, by- 

 means of which this general conception has been carried 

 out. 



Beginning with simple and familiar facts relating to 

 British and European quadrupeds and birds, I have 

 defined and shown the exact character of ''areas of 

 distribution,'' as applied to species, genera, and families, 

 and have illustrated the subject by maps showing the 

 peculiarities of distribution of some well-known groups of 

 birds. Taking then our British mammals and land-birds, 

 I follow them over the whole area they inhabit, and thus 

 obtain a foundation for the establishment of '' zoological 

 regions," and a clear insight into their character as 

 distinct from the usual geographical divisions of the globe. 



The facts thus far established are then shown to be 

 necessary results of the " law of evolution." The nature 

 and amount of "variation'' is exhibited by a number of 

 curious examples ; the origin, growth, and decay of 

 species and genera are traced, and all the interesting 

 phenomena of isolated groups and discontinuous generic 

 and specific areas are shown to follow as logical conse- 

 quences. 



The next subject investigated is the means by which 

 the various groups of animals are enabled to overcome the 

 natural barriers which often seem to limit them to very 

 restricted areas, how far those barriers are themselves 

 liable to be altered or abolished, and what is the exact 

 nature and amount of the changes of sea and land which 

 our earth has undergone in past times. This latter part 

 of the inquiry is shown to be the most important as it is 

 the most fundamental ; and as it is still a subject of 

 controversy, and many erroneous views prevail in regard 

 to it, it is discussed at some length. Several distinct 

 classes of evidence are adduced to prove that the grand 

 features of our globe — the position of the great oceans 



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