[33] The nomenclature of the Australian crayfishes requires thorough revision. I therefore, for the present, assign no name to this crayfish. It is probably identical with the A. nobilis of Dana and the A. armatus of Von Martens.
In addition to these, no crayfish have as yet been found in any of the great rivers, such as the Orinoko; the Amazon, in which they were specially sought for by Agassiz; or in the La Plata, on the eastern side of the Andes. But, on the west, an “Astacus” chilensis is described in the “Histoire Naturelle des Crustacées,” (vol. ii. p. 333). It is here stated that this crayfish “habite les côtes du Chili,” but the freshwaters of the Chilian coast are doubtless to be understood.
Finally, Madagascar has a genus and species of crayfish (Astacoides madagascariensis, fig. [65]) peculiar to itself.
On comparing the results obtained by the study of the geographical distribution of the crayfishes with those brought to light by the examination of their morphological characters, the important fact that there is a broad and general correspondence between the two becomes apparent. The wide equatorial belt of the earth’s surface which separates the crayfishes of the northern from those of the southern hemisphere, is a sort of geographical {310} representation of the broad morphological differences which mark off the Potamobiidæ from the Parastacidæ. Each group occupies a definite area of the earth’s surface, and the two are separated by an extensive border-land untenanted by crayfishes.
FIG. 77.—MAP OF THE WORLD, showing the geographical distribution of the Crayfishes. I. Eur-asiatic Crayfishes; II. Amurland Crayfishes; III. Japanese Crayfishes; IV. Western North American Crayfishes; V. Eastern North American Crayfishes; VI. Brazilian Crayfishes; VII. Chilian Crayfishes; VIII. Novozelanian Crayfishes; IX. Fijian Crayfishes; X. Tasmanian Crayfishes; XI. Australian Crayfishes; XII. Mascarene Crayfishes.
A similar correspondence is exhibited, though less distinctly, when we consider the distribution of the genera and species of each group. Thus, among the Potamobiidæ, Astacus torrentium and nobilis belong essentially to the northern, western, and southern watersheds of the central European highlands, the streams of which flow respectively into the Baltic and the North Seas, the Atlantic and the Mediterranean (fig. [77], I.); A. leptodactylus, pachypus, angulosus, and colchicus, appertain to the Pontocaspian watershed, the rivers of which drain into the Black Sea and the Caspian (I.); while Astacus dauricus and A. Schrenckii are restricted to the widely separated basin of the Amur, which sheds its waters into the Pacific (II.) The Astaci of the rivers of western North America, which flow into the Pacific (IV.), and the Cambari of the Eastern or Atlantic water-shed (V.) are separated by the great physical barrier of the Rocky Mountain ranges. Finally, with regard to the Parastacidæ, the widely separated geographical regions of New Zealand (VIII.), Australia (IX.), Madagascar (XII.), and South America (VI. and VII.), are inhabited by generically distinct groups.
But when we look more closely into the matter, it will {311} be found that the parallel between the geographical and the morphological facts cannot be quite strictly carried out.
Astacus torrentium, as we have seen, inhabits both the British Islands and the continent of Europe; nevertheless, there is every reason to believe that twenty miles of sea water is an insuperable barrier to the passage of crayfishes from one land to the other. For though some crayfishes live in brackish water, there is no evidence that any existing species can maintain themselves in the sea. A fact of the same character meets us at the other side of the Eurasiatic continent, the Japanese and the Amurland crayfishes being closely allied; although it is not clear that there are any identical species on the two sides of the Sea of Japan.