MARINIE GAMMARIDEAJSr AMPHIPODA 43 



lian gammarus stock, for Schellenberg (1937a,b) has included that 

 genus with the "gammarus" group. If South African "gammaruses" 

 and the poorly known Australian members of the family have indeed 

 been the results of separate fresh-water invasions then the southern 

 hemisphere was completely blocked from an overland dispersal of 

 Eurasian gammarids. 



These stocks of "gammarus" in the southern hemisphere may not 

 be so successful competitively against talitroideans as are Palearctic 

 stocks, for another group of talitroideans, the "chiltonias" have 

 evolved in the African and Australia-New Zealand provinces. 



One need not adhere too closely to the thesis that Baikal formed an 

 evolutionary center for the reinvasion of the sea by its products. 

 Baikal and Titicaca may simply be dismissed as habitats open to 

 whatever contiguous marine groups were present. But this also implies, 

 without considering special ecological adaptations of the groups, that 

 if Gammaridae are indeed primitive and represent the survivors of a 

 base stock out of which came talitroideans, that the latter were under- 

 going their evolution on a tropical frontier while the tropicwards 

 dispersal of various marine Gammaridae was stagnating. Talitroids, 

 therefore, penetrated the tropical barrier in the move southward be- 

 fore marine or fresh-water gammarids. The importance of studying 

 ecological stress in the groups is apparent in the limited "later" success 

 of marine gammarids in partially occupying some of those southern 

 limnetic habitats on the tropical fringe. If there was temporal inde- 

 pendence among all these events in Baikal, South America, South 

 Africa, and Australia, and if marine Gammaridae and Talitroidea 

 were already fully developed before any of these limnetic invasions, 

 then one must entertain a strong and independent ecological success 

 of the Palearctic-Nearctic gammarids, for talitroideans have not been 

 able to populate those realms. 



Terrestrial nonaquatic talitroids have never become eminently 

 successful. They occupy no major continent except where they have 

 been locally introduced into gardens. They are confined to Indo- 

 Pacific islands in the tropics, in moist, biotically impoverished environ- 

 ments. Beachhoppers on the strand of large continents have never 

 been able to migrate inland as far as we know. 



Only two groups of Amphipoda thus have any clear relationship to 

 historical events that we have as yet been able to discover and on which 

 we may devote a great deal of refinement with promise of results other 

 than speculation. The meager evidence seems to indicate that Gam- 

 maridae had their major radiation in cold northern climes, either 

 marine or fresh water and that they had not reached either realm of 

 the southern hemisphere before those continents were blocked by some 

 kind of environmental barrier, leaving them open to the Talitroidea. 



285-135 O - 69 



