36 ESSAY ON CLASSIFICATION. Part I. 



species of doves confined to particular islands in the Pacific Ocean; among Reptiles, 

 by the Proteus of the cave of Adelsberg in Carinthia, by the Gopher (Testudo Poly- 

 phemus Auct.) of our Southern States; among fishes, by the Blind Fish (Amblyopsis 

 spelgeus) of the Mammoth Cave. Examples of closely limited Articulata may not be 

 so striking, yet the Blind Crawfish of the Mammoth Cave and the many parasites 

 found only upon or within certain species of animals, are very remarkable in this 

 respect. Among Mollusks, I would remark the many species of land shells, ascer- 

 tained by Professor Adams to occur only in Jamaica,^ among the West India Islands, 

 and the species discovered by the United States Exploring Expedition upon isolated 

 islands of the Pacific, and described by Dr. Gould.^ Even among Radiata many 

 species might be quoted, among Echinoderms as well as among Medusae and Polypi, 

 which are only known from a few localities; but as long as these animals are not 

 collected with the special view of ascertaining their geographical range, the indica- 

 tions of travellers must be received with great caution, and any generalization 

 respecting the extent of their natural area would be premature as long as the coun- 

 tries they inhabit have not been more extensively explored. It is nevertheless true 

 as established by ample evidence, that within definite limits all the animals occurring 

 in different natural zoological provinces are specifically distinct. What remains to 

 be ascertained more minutely is the precise range of each species, as well as the 

 most natural limits of the different faunae. 



SECTION X. 



IDENTITY OF STRUCTURE OF WIDELY DISTRIBUTED TYPES. 



It is not only when considering the diversification of the animal kingdom within 

 limited geographical areas, that we are called upon in our investigations to admire 

 the unity of plan its most diversified types may exhibit ; the identity of structure of 

 these types is far more surprising, when we trace it over a wide range of country, 

 and within entirely disconnected areas. Why the animals and plants of North 

 America should present such a strong resemblance to those of Europe and Northern 

 Asia, while those of Australia are so entirely different from those of Africa and South 

 America under the same latitudes, is certainly a problem of great interest in connec- 



1 Adams, (C. B.,) Contributions to Conchology, ^ Gould, (A. A.,) Mollusks, United States Ex- 



New York, 1849-50, 8vo. A series of iiamphlets, ploring Expedition, under the command of Ch. 

 full of original information. Wilkes, U. S. N., 1 vol. 4to. Philadelphia, 1854. 



