552 GEOGEAPHICAL ZOOLOGY. [part iv. 



hardly be determined with any definiteness. On continents they 

 seem to spread steadily, but slowly, in every direction, checked 

 probably by organic and physical conditions rather than by the 

 barriers which limit the higher groups. Over the ocean they 

 are also slowly dispersed, by some means which act perhaps at 

 very long intervals, but which, within the period of the duration 

 of genera and families, are tolerably effective. It thus happens 

 that, although the powers of dispersal of land-shells and insects 

 are so very unequal, the resulting geographical distribution is 

 almost the opposite of what might have been expected,— the 

 former being, on the whole, less distinctly localized than the 

 latter. 



CONCLUSION. 



The preceding remarks are all I now venture to offer, on the 

 distinguishing features of the various groups of land-animals as 

 regards their distribution and migrations. They are at best 

 but indications of the various lines of research opened up to us 

 by the study of animals from the geographical point of view, and 

 by looking upon their range in space and time as an important 

 portion of the earth's history. Much work has yet to be done 

 before the materials will exist for a complete treatment of the 

 subject in all its branches ; and it is the author's hope that his 

 volumes may lead to a more systematic collection and arrange- 

 ment of the necessary facts. At present all public museums and 

 private collections are arranged zoologically. All treatises, mo- 

 nographs, and catalogues, also follow, more or less completely, the 

 zoological arrangement ; and the greatest difficulty the student 

 of geographical distribution has to contend against, is the total 

 absence of geographical collections, and the almost total want of 

 complete and comparable local catalogues. Till every well- 

 marked district, — every archipelago, and every important island, 

 has all its known species of the more important groups of animals 

 catalogued on a uniform plan, and with a uniform nomenclature, 

 a thoroughly satisfactory account of the Geographical Distribu- 

 tion of Animals will not be possible. But more than this is 

 wanted. Many of the most curious relations between animal 



