OCR Text |
Show 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 represented 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 freshwater 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 Palcemon lacustris (Anchistia migratoria, Heller) of the Lago di Garda is identical with a Prawn now living in the Mediterranean. Again, the My sis 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, inhabited the ocean, and that it had as wide a distribution as Palcemon or Peneeus 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 Homaridae 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 palaeontological 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 Crustaces d'eau douce de Norvege.' In the British Museum there is a species of that especially marine genus Peneeus which is affirmed by the Messrs. Schlagintweit to have been obtained from an affluent of the Sutlej, at the foot of the Himalayas. Peneeus brasiliensis ascends the North-American rivers for long distances (Smith, in Prof. Baird's Report, 1872-72). |