PILSBRY: NON-MARINE MOLLUSCA OF PATAGONIA. 627 



ARCHIPLATA AS AN EVOLUTION-CENTER. 



Wallace in I876 1 showed that the South American fauna is divisible 

 into two subregions which he called the Tropical or Brazilian and the 

 South Temperate or Chilian. He calls attention to the affinities of the 

 Chilian diurnal Lepidoptera and the Carabidae to North Temperate forms. 2 

 Dr. H. von Ihering in numerous papers 3 has recognized the two subregions 

 of Wallace as distinct evolution centers. He concludes that these centers, 

 Archiplata (that is, Patagonia, southern Brazil, Chili and western Peru) and 

 Archibrazil, were long isolated from one another by an arm of the sea. As 

 primitive elements of the Archiplatan fauna he mentions the fresh-water 

 crab sEglea, the genus Parastacus, and the mollusks Diplodon and Chilina. 

 Negative characteristics are found in the absence of the dominant Ama- 

 zonian genera of mussels and Ampullariidcz (which seem to have invaded 

 the La Plata drainage area comparatively lately, probably in the Pliocene) 

 to which many groups of land-snails might be added. 



The geology of the regions involved is so imperfectly known that we 

 have no positive data for or against the hypothesis that an arm of the 

 Cretaceous sea extended across the continent, as von Ihering claims. This 

 is a question only to be settled by geological exploration of the region, 

 which may perhaps show a Cretaceous transgression similar to that which 

 involved eastern Mexico and the region northward in the middle Cretaceous. 

 Yet the fact remains that, so far as molluscan groups are concerned, there is 

 but little evidence of such an isolation of the Archiplatan area. The barriers 

 to migration imposed by climate have not been taken into account. The 

 AmpitllariidcB are snails that have never, in any region, been able to extend 

 beyond a subtropical climate. TheGb&Kttfe (fig. 36) are apparently, like 

 the large Lymnaeas in North America, snails which cannot exist in a sub- 

 tropical or even a warm temperate environment, however favorable maybe 

 the conditions of migration. It is instructive, in this connection, to com- 

 pare the Lymnaeid faunas of Minnesota and Arkansas, which show great 



1 Geographical Distribution of Animals, II, frontispiece and Chapter XIV. 



1 Dr. Scharff has suggested an explanation of this peculiarity (American Naturalist, Septem- 

 ber, 1909, p. 5 13), but his hypothesis explains only a few facts. It would involve us in problems 

 more intricate than those which it solves. Possibly the systematic relations of the insects in 

 question have not been rightly estimated. 



3 The more important of these articles have been reprinted in his " Archhelenis und Archi- 

 notis," Leipzig, 1907. 



