CEAYFISHES IN THE SOUTH-WEST 219 



ones, yet, like Dr. Ortmann, they maintained that Europe was 

 supplied with its crayfish fauna from the East. In his lucid 

 essay on this subject, Dr. Ortmann argues that a primitive 

 group of Potamobiidae, ancestral to all the living ones, must 

 formerly have existed in eastern Asia, which region should 

 be regarded as the centre of dispersal of the family. This 

 ancient group, he thinks, sent one branch westward to Europe 

 and another eastward across the old Bering Strait land bridge 

 to western North America. Thus, three centres of dispersal 

 gradually originated in which the old stock developed on 

 independent lines. The middle one changed to Cambaroides, 

 jwhile the two branches retained the ancient characters. From' 

 the American branch eventually originated Cambarus, which 

 spread eastward into the eastern States of North America 

 (see pp. 289 91). 



I accepted Dr. Ortmann 's explanation in my work on Euro- 

 pean animals as an hypothesis, which satisfactorily accounted 

 for the present distribution of the Potamobiidae. Doubts, 

 however, have since arisen in my mind as to whether there 

 is not a better theory. The more I studied the problem the 

 less I felt disposed to agree with Dr. Ortmann's explanation. 

 Why should the old stock, for instance, have become modified 

 into Cambaroides in its original centre of dispersal, while still 

 flourishing in two centres enormously distant from one 

 another ? And these two new centres were reached after many 

 struggles and vicissitudes, after long and weary travels, prob- 

 ably through hundreds of miles of unsuitable ground. One 

 would imagine the two distant branches to have become 

 more and more unlike one another. Five species of the 

 old stock Potamobius still inhabit the streams of western 

 America, from California in the south to Alaska in the north. 

 If Dr. Ortmann's theory were the correct one, the centre of 

 dispersal of the more modern genus Cambarus, which has 

 developed from some member of the old stock, ought to be in 

 north-western America. Everything, nevertheless, points to 

 the conclusion that the new genus Cambarus originated in 

 Mexico, and Dr. Ortmann (p. 291) is of that opinion, having 

 recently supported it by means of many additional facts of 

 distribution.* 



* Ortmann, A. E., "Affinities of Cambarus." 



