MATTHEW, CLIMATE AND EVOLUTION 301 



in our deductions from the evidence afforded by the various classes, it 

 appears to me that we should hold to conservative views rather than 

 adopt hypotheses of continental relations so much at variance with gen- 

 erally accepted geological principles and inferences. 



To illustrate the point that these discrepancies are a matter rather of 

 interpretation than data I may venture to discuss one or two instances 

 among invertebrates prominently used in paleogeography. 



INTERPRETATION OF DISTRIBUTION DATA OF CRAYFISH 



I am indebted for my data on this interesting group to Dr. Ortmann's 

 valuable discussion of the geographical distribution of fresh-water 

 Decapoda.^^^ The interpretation, however, which I would place upon 

 the facts differs widely from his. 



As Professor Huxley has observed, the real difficulty in explaining the 

 distribution of the crayfish is in their occurrence in the north and south 

 temperate zones, separated by a wide tropical belt in which none now 

 occur or are known to have occurred in the past. Two explanations offer 

 themselves : 



1) Independent adaptation from marine types in the northern and 

 southern hemispheres. This would involve either former Antarctic con- 

 nections or independent adaptation also of the several southern groups 

 from marine types. 



2) Former cosmopolitan distribution of crayfish, with subsequent dis- 

 appearance from the tropical belt and differentiation of the isolated south- 

 ern groups and of the more progressive northern groups. 



The latter view is generally accepted, and seems to me more consonant 

 with the facts of distribution, e. g., presence of crayfish in Madagascar, 

 while they are absent from South Africa. I am unable to agree with 

 Dr. Ortmann that crayfish on oceanic islands necessarily involve a former 

 land connection, since such land connections as he finds it necessarv to 

 postulate would apparently involve the presence on these islands of con- 

 tinental faunge which are not now present, and whose absence cannot be 

 reasonably accounted for. For the reasons already presented I see no 

 difficulty in supposing that the crayfish of Cuba, Madagascar, New 

 Zealand or Fiji have reached those islands by accidental transport of 

 natural "rafts" through the agency of ocean currents, or by other acci- 

 dental means. The Australian and South American crayfish I should 

 regard as derived from the north, by way of the existing or slightly sub- 



^ A. E. OnTMANN : "Geographical Distribution of Fresh-water Decapods and its Bear- 

 ing upon Ancient Geography," Proc. Amer. I'hil. Soc. vol. xli, pp. 267-400. 1902. 



