BIOGEOGRAPHICAL PROVINCES 1061 



Ortmann, forming a morphologic equivalent (homoeomorph) of 

 Cambariis, though not closely related to it. Ortmann believes that 

 the three groups originated from a common ancestor whose home 

 was in eastern Asia. A branch was sent out westward, which finally 

 reached Europe ; while another branch migrated eastward, reaching 

 western America by means of a land area connecting Asia and 

 America. Of these migrants a single species has remained behind 

 in Unalaska. When finally, by geographical changes, the European 

 and American branches became separated from the Asiatic one, 

 each developed independently, the result being three distinct groups, 

 as above stated. From members of the American group, which had 

 reached Mexico, the genus Cambarus developed, in a remote period, 

 becoming differentiated into five groups, through processes of geo- 

 graphical isolation. 



From the distribution of these and other fresh water decapods 

 Ortmann concludes that the following land connections must have 

 existed in the near past. 



1. Northeast Asia with Northwest America across the Behring 



Sea. 



2. East Asia with Australia. 



3. South Asia with Madagascar and Africa. 



4. New Zealand with Australia. 



5. Australia with South America. 



6. West Indies with Central and South America. 



7. South America with Africa. 



Ortmann holds that the above connections were necessary ; for, 

 while a few species of fresh water crayfish or crabs have been 

 found in brackish or even salt water, this occurrence is very excep- 

 tional, the animals being preeminently dwellers in fresh water, so 

 that a migration across oceans or parts of oceans is practically pre- 

 cluded. Furthermore, since these animals cannot live out of 

 water for any great length of time, deserts or waterless tracts form 

 absolute barriers for them. The eggs of these creatures are carried 

 under the abdomen of the female, and the young hatch in a state 

 similar to the parents. While water fowl or other agents may oc- 

 casionally effect a passive transport, such cases are rare and have 

 never been observed. "The whole character of the distribution of 

 the different species is against the assnuiption of exceptional tneans 

 of dispersal." (Ortmann-39.) The connection of Asia and north- 

 west America by way of Behring Sea is also indicated by the dis- 

 tribution of mammals and other land animals. The connection con- 

 tinued probably through the whole of Tertiary time. The connec- 



