ORIGIN OF LIFE IN AMERICA 
426 
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, hut 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, hut- 
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. Subsequently, either during Cretaceous 
or early Tertiary times, a Pacific Continent must have united 
Hutton, F. W., “ Ancient Antarctica,” p. 245. 
