CRAYFISHES 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, 
while 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 tlie 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.” 
