Wagner, Huxley, and Wallace 327 
the rescue with his Darwin’sche Theorie und das Migrations-Gesetz 
der Organismen’. 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 re- 
stricted 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 subse- 
quent 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 late to introduce Wallace’s name in 1876, the year 
of the publication of his standard work?, We cannot do better than 
quote the author’s own words, expressing the hope that his “book 
should bear a similar relation to the eleventh and twelfth chapters 
of the Origin of Species as Darwin’s Animals and Plants under 
Domestication does to the first chapter of that work,” and to add 
that he has amply succeeded. Pleading for a few primary centres he 
accepts Sclater’s six regions and does not follow Huxley’s courageous 
changes which Sclater himself had accepted in 1874. Holding the 
* Leipzig, 1868. 
2 The Geographical Distribution of Animals, 2 vols. London, 1876, 
