218 
ORIGIN OF LIFE IN AMERICA 
their dispersal has been caused by any exceptional or abnormal 
means of transport. Few zoologists will consequently dis¬ 
agree with Dr. Ortmann’s * assertion that fresh-water cray¬ 
fishes are among the most important animals in so far as the 
study of their distribution elucidates past changes of land 
and water over the globe. The crayfishes have been brought 
into great prominence by Professor Huxley’s well-known 
treatise on the subject. More recently it is principally in 
America that their structure and distribution have been 
studied with great assiduity. Dr. Faxon f was the first to 
recognise that besides the American genera Potamobius 
(Astacus) and Cambarus, there is a third genus of fresh¬ 
water crayfishes which inhabits north-eastern Asia. The 
latter, it is true, is only considered a sub-genus of Pota¬ 
mobius by Dr. Faxon and also by Dr. Ortmann, but, as 
Mr. Stebbing ^ has pointed out, its intermediate position 
between Potamobius and Cambarus entitles it to rank as the 
distinct genus Cambaroides. 
The geographical distribution of these crayfishes (Pota- 
mobiidae) is very suggestive and interesting. Europe is the 
headquarters of the old and well-known genus Astacus, which 
name, in the unfortunate search for priority, has had to 
give way to Potamobius. The genus ranges practically 
throughout Europe, from north to south and from east to 
west, and only very little beyond it. Beyond the Caucasus 
it crosses into Transcaucasia, Turkestan and western Siberia. 
It is quite unknown in the remainder of Asia. In the rivers 
of eastern Asia, in Korea, Japan and eastern Siberia we meet 
only with members of the small group Cambaroides referred 
to. The somewhat close resemblance of this Asiatic genus 
to the American Cambarus does not point to blood relation¬ 
ship, according to Dr. Ortmann, merely to convergence. In 
America we find not only Cambarus, but also Potamobius, the 
European crayfish, the latter genus being in America entirely 
confined to the western States. Professor Huxley and Dr. 
Faxon both urged that the American species of Potamobius re¬ 
sembled the European crayfishes much more than the Asiatic 
* Ortmann, A. E., “ Distribution of Freshwater Decapods,” pp. 315 —316.' 
f Faxon, W., “ Eevision of Astacidae.” 
\ Stebbing, T. E. E., “ Crustacea,” p. 208. 
