, and this view agrees perfectly with all
the details of distribution. The centre of distribution lies in Northern
Asia, or in Arctic North America. From there the great genus _Lagopus_
spread east and west, reaching Europe by these vastly divergent routes
at a time when the physical geography was very different from what it is
to-day. Several of the species common to the Alps and Scandinavia have
migrated from Siberia direct to Eastern Europe. But we can now imagine
how from a similar centre in Asia--perhaps at a rather more remote
time--a species spread eastward across North America and Greenland to
Scandinavia, and westward along the mountain ranges of the Tian Shan and
the mountains of Asia Minor to Greece, and finally to the Alps. We
should then have the same species in the Alps and in Scandinavia, not
far removed from one another; but how different were their paths of
migration! This, however, is not an imaginary instance. Such a migration
must have actually taken place in a good number of instances among the
terrestrial invertebrates and also among plants.

The view still current among many zoologists and botanists, that animals
and plants were driven down into the plain from the mountains of Europe
during the height of the Glacial period and there lived together till
the return of a more genial temperature, when they retreated to their
mountain homes, is a very plausible one. During their sojourn in the
plain, the plants and animals--say from Scandinavia--intermingled with
those from the Alps; and when the time of separation came, many Alpine
forms retired northward with the Scandinavians, while many Scandinavians
would go with the Alpines to their home. In this way the similarity
between the Alpine and Scandinavian faunas and floras is assumed to have
been brought about. These theories, first promulgated by Edward Forbes,
were hailed with general satisfaction by the scientific world. Even
Darwin says of them (p. 331), that grounded as they are on the perfectly
well-ascertained occurrence of a former Glacial period, they seemed to
him to explain in a satisfactory manner the present distribution of the
Alpine and Arctic productions of Europe. To the present day this view
meets with much favour among naturalists. It is somewhat similar to one
which has recently been strongly supported by Professor Nehring and
accepted by Professor Th. Studer and many others. They have never made
it quite clear whether the pre-glaci