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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 arrangement
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 wellmarked
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