1878.] AND DISTRIBUTION OF THE CRAYFISHES. 787 



Africa and Asia south of the great Asiatic highlands, just as the 

 Crayfishes are. It will be very interesting to learn, from the thorough 

 investigation of the fauna of Madagascar, which is now being carried 

 out, whether the Salmonoids or their allies are in any way repre- 

 sented there. 



The broad similarity in distribution between the Salmoniform 

 fishes and the Crayfishes is doubtless due to the likeness of the 

 conditions under which the two groups have reached their present 

 development. I do not think that there can be any reasonable 

 ground for questioning the assumption, that both the freshwater 

 fishes and the freshwater Crustacea are modifications of a marine 

 prototype, which has more or less completely adapted itself to fresh- 

 water conditions. In the case of the Crayfishes, at any rate, there 

 is abundant analogical evidence in support of this hypothesis. It is 

 well known that, in many parts of the world, the Prawns ascend 

 rivers, and become fluviatile. The Palamon lacustrls (Anchistia 

 migratoria, Heller) of the Lago di Garda is identical with a Prawn 

 now living in the Mediterranean. Again, the Mysis relicta of the lakes 

 of Norway, Sweden, Western Russia, and North America (Michigan 

 and Superior) is only a variety of the Mysis oculata of the Arctic 

 seas '. Nor do I think it can be seriously questioned that the 

 fluviatile and the land Crabs are modified descendants of marine 

 Brachyura. 



Let it be supposed that, at some former period of the earth's 

 history, a Crustacean, similar to Paranephrops or Astacopsis in its 

 general characters, but with the first pair of abdominal appendages 

 fully formed, which we may call provisionally Protastacus, in- 

 habited the ocean, and that it had as wide a distribution as Palcemon 

 or Penceus have at the present day. Let us suppose, further, that 

 the northern form of the genus tended towards the assumption of 

 the Potamobiine, and the southern towards that of the Parastacine 

 type. Under these circumstances, it is easy to understand how such 

 rivers as were, or became, accessible in both hemispheres, and were 

 not already too strongly tenanted by formidable competitors, might 

 be peopled respectively by Potamobiine or Parastacine forms, which, 

 acquiring their special characters in each great river-basin, would 

 bring about the distribution we now witness. As time went on, the 

 Protastacus stock might become extinct, or might be represented 

 only by rare deep-water forms, as the Homaridse are represented in 

 the Indian Ocean only by Nephropsis. 



Some such hypothesis as this appears to me to be fully justified by 

 the present state of knowledge ; and though it cannot as yet be said 

 to be directly supported by palseontological facts, these facts agree with 

 the hypothesis very well as far as they go. For the Mesozoic marine 



1 G. O. Sars, ' Histoire Naturelle des Crustacea d'eau douce de Norvege.' 

 In the British Museum there is a species of that especially marine genus 

 Penieus, which is affirmed by the Messrs. Schlagintweit to have been obtained 

 from an affluent of the Sutlej, at the foot of the Himalayas. Peiwus brasilienm 

 ascends the North-American rivers for long distances (Smith, in Prof. Baird's 

 Report, 1872-72). 



