Wagner, Huxley, and Wallace 327 



the rescue with his Darwin'sche Theorie nnd das Migrations-Gesetz 

 der Organismen 1 . 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 2 . 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 



1 Leipzig, 1868. 



2 The Geographical Distribution of Animals, 2 vols. London, 1876. 



