OCR Text |
Show 204 DR. W. T. CALMAN ON THE MACRUROUS [Mar. 6, pp. 57-134, 1905) that certain species occasionally present " mutations" leading at a single step from Caridina to Ortmannia and from Ortmannia to Atya, must be borne in mind as suggesting the possibility that the Atya-\ike characters of Atyella m a y have arisen independently in the Tanganyika forms. In any case, there can be no doubt that the Atyidae of Tanganyika rank among the most highly specialised members of the family and are far removed from such primitive forms as Xiphocaris and Atyaephyra. W h e n describing the two species of Prawns discovered by Mr. Moore in Tanganyika, I pointed out (Proc. Zool. Soc. 1899, p. 711)* that they threw no light on the general question of the origin of the Tanganyika fauna, inasmuch as they belong to groups which are characteristically inhabitants of fresh-water. Since then, in his book on ' The Tanganyika Problem' and elsewhere, Mr. Moore has claimed that the prawns belong to the " relict," or as he terms it " halolimnic," section of the fauna of that lake. H e believes that the members of this section are distinguished by special resemblances to marine forms and by generally primitive characters. He supposes that they represent the descendants of marine species which reached their present habitat not later than the Jurassic epoch, when the present site of the lake was occupied by an arm of the sea. It is necessary, therefore, to state definitely that there is not the smallest ground for supposing that the Macrurous Crustacea of Tanganyika have had such an origin. The groups to which they belong, the genus Pcdcemon and the family Atyidae, are widely distributed in the fresh-waters of tropical regions, and the fact that representatives of both occur in Tanganyika is, in itself, no more surprising than the fact that representatives of both'occur in the Upper Nile. Nor is it the case that the Tanganyikan species present such primitive characters as would brine them closer to the hypothetical marine stocks from which these groups have arisen. As regards the Atyidae, at all events, the reverse is the case, for the Tanganyikan genera are in some respects the most specialised members of the family. What does distinguish the Macruran fauna of Tanganyika is the great number of species found within a limited and continuous area t and their distinctness, so far as we know, from all the species inhabiting adjacent regions. The explanation of these peculiarities is a very difficult problem and one which cannot be profitably considered apart from the similar problems presented by the other elements of the Tanganyikan fauna. For the present, however, * In stating (I.e.) that the genus Caridina was not known to occur in West Africa, I overlooked Hilgendorf's description (SB. Ges. naturf. Freunde Berlin, 1893, p. 156) of a species from Togoland. Bouvier has since recorded a variety ot the same species from the interior of the French Congo and from the neighbourhood of Lake Tchad. f It has lately been suggested by Dr. F. Sarasin (C. R. Congres Internat. Zool. Berne, 1904 (1905) p. 151) that the peculiar richness in Decapod Crustacea which distinguishes the fresh-waters of Celebes may be directly correlated with the poverty of the fish-fauna of that island. It is plain that this explanation cannot be applied to the case of Tanganyika, where the fish-fauna is remarkably rich. |