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GU GEOGRAPHICAL ZOOLOGY. ' [part iv.
uniformity of organisation. One family, which continued to
exist in Europe till the latter part of the Miocene period,
reached America, and has there been preserved to our day.
Lines of Migration of the Mammalia.—The whole series of
phenomena presented by the distribution of the Mammalia,
looked at broadly, are in harmony with the view that the great
continents and oceans of our own epoch have been in existence,
with comparatively small changes, during all Tertiary times.
Each one of them has, no doubt, undergone considerable modifi-
cations in its area, its altitude, and in its connection with other
lands. Yet some considerable portion of each continent has,
probably, long existed in its present position, while the great
oceans seem to have occupied the same depressions of the
earth's crust (varied, perhaps, by local elevations and sub-
sidences) during all this vast period of time. Hence, allowing
for the changes of which we have more or less satisfactory
evidence, the migrations of the chief mammalian types can be
pretty clearly traced. Some, owing to their small size and
great vitality, have spread to almost all the chief land masses
but the majority of the orders have a more restricted range.
All the evidence at our command points to the Northern
Hemisphere as the birth-place of the class, and probably of all
the orders. At a very early period the land communication
with Australia was cut off, and has never been renewed ; so
that we have here preserved for us a sample of one or more
of the most ancient forms of mammal. Somewhat later the
union with South America and South Africa was severed;
and in both these countries we have samples of a somewhat
more advanced stage of mammalian development. Later still,
the union by a northern route between the Eastern and Western
Hemispheres appears to have been broken, partly by a physical
separation, but almost as effectually by a lowering of tempera-
ture. About the same period the separation of the Palsearctic
region from the Oriental was effected, by the rise of the
Himalayas and the increasing contrast of climate; while the
formation of the great desert-belts of the Sahara, Arabia,
Persia, and Central Asia, helped to complete the separation of
;