GEOGRAPHICAL DISTRIBUTION. 177 



the one hand, and between those of Eastern North America and of Eastern Asia on the other, 

 I have simply made a new application of the theories advanced by Huxley* to explain 

 the differences between the crayfishes on the two sides of the equator, and by Asa Gray J 

 to account for phenomena in the distribution of plants similar to those presented by the 

 crayfishes of the Northern hemisphere. 



The absence of Astacidae over a large part of Asia is well known. None are found in 

 the great rivers that flow into the Arctic Ocean, nor in those of the central and southern 

 portions of the continent. In connection with the absence of crayfishes from the rivers 

 uf Southern Asia, Milne Edwards* has suggestively observed that these waters are popu- 

 lous with fluviatile crabs of the family Telphusidte. Indeed, as a general rule, crayfishes 

 are unknown in regions where fluviatile crabs abound, having succumbed, perhaps, to their 

 more highly organized rivals. Huxley remarks, moreover, that if the western Eurasiatic 

 crayfishes are derived from a primitive Aralo-Caspian stock, as seems probable, the great 

 Asiatic highlands would form an obstacle to their southward extension into India, while 

 the severity of the Siberian winter and the recent submergence of the land beneath the 

 ocean are invoked to account for the absence of these animals from the great Asiatic rivers 

 that empty into the Arctic Ocean. 



IV. The only islands in the Northern hemisphere known to be inhabited by crayfishes 

 lie near the mainland, and the crayfishes contained therein are either the same species as 

 those of the adjacent part of the continent, or closely related species. Thus, the species 

 found in England and Ireland and in the islands of Cherso and Veglia are the same as 

 those of the western and southern parts of Continental Europe, viz. Astacus pallipes. The 

 Japanese crayfishes {Cambaroidcs Japonicus) are nearly related to those of the Amoor 

 River (Cambaroidcs Dauricus and C. Schrenckii), the Cuban species (Cambarus Cubensis) 

 to those of Mexico (0. Mexicanus). The chances in favor of accidental transportation of 

 animals having the habits of crayfishes across bodies of salt water such as separate the 

 islands in question from the continents are so small, that it seems more probable that 

 their distribution was effected through migrations at a former period, when the present 

 insulated areas were continuous with the neighboring continents. The connection of the 

 British Isles with the continent of Europe in post-glacial times is admitted by geologists. 

 Evidence pointing to the former connection of the islands of the West Indian archipelago 

 with each other and with the mainland has been obtained already from the land fauna 

 and flora of these islands. § 



V. Blind crayfishes have been found in the caves of Carniola and the United States. 

 The Carniola blind crayfish is not merely specifically, but even generically, distinct from 

 the other species of Europe, and belongs to the same genus as the crayfishes of the At lant ic 

 slope of North America (Cambarus). As the genus Cambarus in North America was not 

 developed under the influences affecting cavern life, it would seem that the generic iden- 

 tity of the Carniola cave species with the North American forms cannot be due to simi- 

 larity of surroundings, but rather to genetic connection. In other words, it is probable 



* Op. cit. 



-|- Mem. Amer. Acad. Arts and Sci., New Series, VI. 377-452, 1S57- — Proc. Amcr. Assoc. Adv. Sci., 

 21st Meeting, pp. 1-31, 1873. 



% Histoire Naturelle des Crustaces, III. 584, 1840. 



§ Cf. bland, Proc. Amer. Pliilosopli. Soc , XII. 56, ls;i ; Ann. bye. Nat. Hist. N. Y., X. 311. L874S 

 Am, \ J An.nl. SoL, II. 117, 1880. Eggers, Bull. U. s. Nat. Mns,, No. L3, 1879. The extinct fauna of 

 Cuba includes a giant sloth, Blegalonyx (Leidy, Proc. Acad. Nat. Sci. Phila., 1868, p. 178); and in the little 

 island of Anguilla, which is onlj thirtj five square miles in area, are found the fossil remains of several species 

 of gigantic rodents and a deer (Cope, Proc. Amer. Philosopb. Soc, XI. L83, L869; Lbid , \1. 60S, 1870). 



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