52 PROCEEDIXGS OF TllK 



Finallv, Osborn describes the hypothetical reconstruction of 

 Antarctica as " one oF the greatest triumphs of recent biological 

 investigation " ('The Age of Mammals,' 1910, p. 75). * 



2 . Argument. 



The distribution records of recent antl fossil species upon which 

 the generalisations of the foregoing authors depend have never 

 been denied. Indeed, they continue to increase with the progress- 

 of science. 



To other, and usually earlier, authors these views presented 

 two insuperable diflSculties. One is the extreme change in climate 

 which formerly permitted temperate and subtropical animals and 

 plants to exist where cold is now so intense. The other is the 

 demand for the existence of Tertiary land where an ocean no\\- 

 extends so broad and deep as that between Antarctica and 

 Tasmania or New Zealand. 



To evade these difficulties and yet explain existing distribution, 

 the following three alternatives have been advanced. 



That decadent groups were expelled from their original seats 

 by more vigorous competitors : retreating from a northern centre 

 to the ends of the earth, such groups divided into fugitive parties 

 which converged as southern lands approached the pole. Or 

 discontinuous distribution in southern continents were simply 

 considered remnants of a former universal distribution (Wallace, 

 'The Geographical Distribution of Animals,' i. 1876, p. 39S; 

 Pfeffer, Zool. Jahrb. Suppl. viii. 1905, pp. 407-442). 



But whereas, under the circumstances postulated, the northern 

 wanderers would be expected to diminish and to vaiy as they 

 receded, the southern forms in question became more alike and 

 more numerous proceeding south. Thus radiation rather than 

 convergence is indicated. 



II. 



That birds, winds, or circumpolar currents, by a process of 

 picking up and setting down passengers from the continents or 



* Wliile this article was in the press, there reached rue an imporUmt 

 memoir by Dr. Pilsbry on " The Non-Marine Molhisca of Patagonia " (Rep. 

 Princeton Univ. Exped. Patagonia, iii. 1912, pt. v. pp. ol3-633). My friend 

 considers Antarctica rather as a road for migration, especially an American 

 exit, than as a centre of evolution. Ho t^akes exception to my deriva- 

 tion of Australian Acavida^ from Antarctica, and suggests that the group 

 arose in Gondwana Land. On reconsideration I would still maintain tliat the 

 south-eastwardly increasing distribution of Australian Acavida^ indicates their 

 immediate Antarctic origin. But previous to an Antarctic sojourn the group 

 may have been Gondwana bred. This memoir heightens the resemblance 

 between ea.st and west. Gundlachia, Liphdon, and Radiodiscus are common, 

 Pettcrdia7m scarcely differs from Littoridina, and I'ofamolithis appears to have 

 Tasmanian relatives. 



