426 ORIGIN OF LIFE IN AMERICA 



the rivers of western and southern South America. No living 

 Diplodon has ever been discovered in Central or North 

 America, where other Unionidae are found in the greatest pro- 

 fusion. No Diplodon has ever been discovered fossil either in 

 Central or North America, although a great many fossil 

 Unionidae are known to science. Hence it seems certain that 

 Diplodon has never lived north of its present habitat. Yet 

 in New Zealand, in Tasmania and Australia this genus re- 

 appears. Further north in Asia it has never been met with. 

 Some zoologists are of opinion that affinities such as the one 

 alluded to can be interpreted by the supposition of a former 

 sub-universal distribution and a subsequent extinction in all 

 but the present habitats. We do not possess a shadow) of any 

 evidence for such a belief, in so far as the range of the fresh- 

 water mussel Diplodon is concerned. 



Let us take the second case, that of the fresh-water cray- 

 fishes. Crayfishes are abundant in the streams of North 

 America and Asia, but they all belong to the family Potamo- 

 biidae, while southern South America is inhabited by quite a 

 different family, the Parastacidae. Crayfishes of the latter 

 family are again met with on the other side of the Pacific, but 

 only in the extreme south, in New Zealand, Tasmania and Aus- 

 tralia. Neither of these instances can be due to convergence, 

 nor to a passage from one continent to the other by way of 

 the northern continents. A direct land bridge becomes abso- 

 lutely essential, yet this need not necessarily have lain in the 

 direction of the antarctic regions. Its position might have been 

 further north, as suggested by Professor Hutton,* although 

 the latter has more recently revised his theories, in so far 

 as he now advocates two south Pacific land bridges, instead 

 of a single one as previously maintained. In 1905 he 

 announced that, having reconsidered his former conclusions, 

 he believed that an Antarctic Continent existed in Jurassic 

 times which connected South America with New Zealand and 

 South Africa. He thinks that this continent sank in the Cre- 

 taceous Period, Antarctica never having since been connected 

 with northern lands. Su,bsequently, either during Cretaceous 

 or early Tertiary times, a Pacific Continent must have united 



* Hutton, F. W., " Ancient Antarctica," p. 245. 



