ished itself, and
a new species c which arose through separation out of the same ancestral
a, which itself persists as such unaltered. Von Buch's prophetic view
seems to have escaped Lyell's and even Wagner's notice.) came to the
rescue with his "Darwin'sche Theorie und das Migrations-Gesetz der
Organismen". (Leipzig, 1868.) He shows that migration, i.e. change of
locality, implies new environmental conditions (never mind whether
these be new stimuli to variation, or only acting as their selectors or
censors), and moreover secures separation from the original stock and
thus eliminates or lessens the reactionary dangers of panmixia. Darwin
accepted Wagner's theory as "advantageous." Through the heated polemics
of the more ardent selectionists Wagner's theory came to grow into an
alternative instead of a help to the theory of selectional evolution.
Separation is now rightly considered a most important factor by modern
students of geographical distribution.

For the same year, 1868, we have to mention Huxley, whose Arctogaea and
Notogaea are nothing less than the reconstructed main masses of land
of the Mesozoic period. Beyond doubt the configuration of land at that
remote period has left recognisable traces in the present continents,
but whether they can account for the distribution of such a much later
group as the Gallinaceous birds is more than questionable. In any case
he took for his text a large natural group of birds, cosmopolitan as
a whole, but with a striking distribution. The Peristeropodes, or
pigeon-footed division, are restricted to the Australian and Neotropical
regions, in distinction to the Alectoropodes (with the hallux inserted
at a level above the front toes) which inhabit the whole of the
Arctogaea, only a few members having spread into the South World.
Further, as Asia alone has its Pheasants and allies, so is Africa
characterised by its Guinea-fowls and relations, America has the Turkey
as an endemic genus, and the Grouse tribe in a wider sense has its
centre in the holarctic region: a splendid object lesson of descent,
world-wide spreading and subsequent differentiation. Huxley, by the way,
was the first--at least in private talk--to state that it will be for
the morphologist, the well-trained anatomist, to give the casting vote
in questions of geographical distribution, since he alone can determine
whether we have to deal with homologous, or analogous, convergent,
representative forms.

It seems