BIOGEOGRAPHICAL PROVINCES 1059 



To these may be added the Antarctic continent, or Aiitarctocjira, the 

 former existence of which, connecting most of the other regions, 

 seems to be demonstrated by a number of independent Hnes of evi- 

 dence, as developed in the study of the distribution of a great 

 variety of types of organisms. 



Fresh Water Biotic Provinces. 



Simpson (47) lias proposed the following regions as subdivisions 

 of the fresh water realm, based on the distribution of the fresh 

 water mussels or L'nios in the present chronofauna. 



1. Palfearctic. 5. Neotropic. 



2. Ethiopic. 6. Central American. 



3. Oriental. 7. Mississippian. 



4. Australian. 8. Atlantic. 



The Pahcarctic region comprises Europe, North Asia, and 

 western North America ; the Oriental includes the Malaysian Is- 

 lands to New Guinea. Simpson holds that the mussels originated in 

 North America in the Triassic, whence they migrated or dispersed 

 into South America, from which region they passed by an old 

 Antarctic land bridge to New Zealand and Australia, thence to 

 southern East Asia, whence they entered the remaining part 

 of Asia, Europe and Africa. In early Tertiary time they migrated 

 across northeast Asia to northwest America. 



It should be noted in this connection that the active migration 

 of the adult is a comparatively slow process, and is, of course, con- 

 fined to streams. The young are attached to the gills of fishes and 

 so are much more rapidly distributed. Here, too, of course, con- 

 tinuous water bodies are required. But there are other methods 

 of dispersal by means of which these shells can transgress land 

 barriers often of considerable extent. Notable among these are 

 the dispersal of the eggs and young shells by water birds, to whose 

 feet they become attached, and by large water beetles and other 

 insects. Thus, while large bodies of sea water cannot be traversed 

 in this way, narrow straits could easily be covered. 



Perhaps a better index to the former land connections is found 

 in the distribution of the fresh water crayfish. Their migrations 

 were traced to a certain extent by Huxley (24), later by Faxon, 

 and most recently by Ortmann (39). 



The distribution of the family Potamohiidce Huxl., compris- 

 ing two genera, Cambarus and Potamobius, may be briefly con- 



