CHAP. XXIV.] SUMMARY AND CONCLUSION. 



501 



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 dis- 

 continuous generic and specific areas are shown to follow as 

 logical consequences. 



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 pre- 

 vail 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 and 

 the chief land-areas — have remained, on the whole, unchanged 

 throughout geological time. Our continents are shown to be 

 built up mainly of " shore-deposits ; " and even the chalk, which 

 is so often said to be the exact equivalent of the globigerina- 

 ooze " now forming in mid-Atlantic, is shown to be a com- 

 paratively shallow-water deposit formed in inland seas, or in 



