552 
GEOGRAPHICAL 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 
