40 ESSAY ON CLASSIFICATION 



would remark the many species of land shells, ascertained by Pro- 

 fessor 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 Medusas 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 geographi- 

 cal range, the indications of travelers must be received with great 

 caution, and any generalization respecting the extent of their natural 

 area would be premature as long as the countries they inhabit have 

 not been more extensively explored.^^ It is nevertheless true as es- 

 tablished by ample evidence, that within definite limits all the ani- 

 mals occurring in different natural zoological provinces are specifi- 

 cally 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 faunas. ^^ 



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 exhibit; the identity of structure of these types is far more sur- 

 prising when we trace it over a wide range of country and within 



^^ Charles B. Adams, Contributions to Conchology (12 nos., New York, 1849-1852). 

 A series of pamphlets, full of original information. 



^ Augustus A. Gould, Mollusca and Shells, United States Exploring Expedition, Re- 

 ports, XII (Philadelphia, 1852). 



" With reference to the Echinoderms and Acalephs, I am able to state that the 

 species of the Atlantic shores of North America, found along the northern states, differ 

 entirely from those of the southern states, and these differ again from those of the 

 Gulf of Mexico. 



^ [Agassiz relies heavily on this argument and returns to it time and again, because 

 the concept of the separate and independent creation of species was essential to his 

 denial of common parentage and consequent genetic unity in animals. He therefore 

 had to insist that flora and fauna inhabited limited and distinct zones of creation. An 

 evolutionist would interpret such zones as ecological boundaries that encouraged the 

 variation of species from a common stock.] 



