484 Recent freshicater Molhisca, 6^c. 



under modified habits, in distant localities, may produce a 

 variation which, under the circumstances, would be consid- 

 ered specific, no intervening links being possible ; so that, 

 after all, such species may be founded upon quasi characters,* 

 just as two mollusks have been described under separate 

 names, because one lives in rough water, and has a strong 

 shell, and the other in smooth water, with a thin shell, when 

 the varying texture of the shells has depended upon the local- 

 ity.! Now, as most of the species named in my list do not 

 present appreciable distinctions, although found in regions 

 widely separated, it remains for those who contend that local- 

 ities indicate species, to inform us how far west of its present 

 position Ireland should have been placed, to induce them to 

 believe its Paludina vivipara another species ; and, in case 

 this island were to occupy the middle of the Atlantic, whether 

 the analogous shell should be considered as appertaining to 

 the American, the European, or to an entirely distinct species. 

 The same question might be raised with respect to Rhinoceros 

 sondiacus, found equally in Sumatra and Java ; and Elephas 

 maximus, because it inhabits some of the Asiatic islands, as 

 well as the continent. Mr. Jenyns is inclined to doubt the 

 validity of " many of the species found in distant latitudes, 

 which, although presenting the closest affinity to some in 

 Europe, have been considered distinct by naturalists, princi- 

 pally on the ground of their inhabiting such different geograph- 

 ical positions." — Mag. Zool. Bot., i. 25. 



* They may be what Dr. Engelmann calls geographical varieties, " where no spe- 

 cific distinction can he discovered between the natives of both continents, but where 

 the American and European variety can always be distinguished by some points of 

 minor importance.'' — Am. J. Sci., vol. xlvi. p. 94. According to the same botanist, 

 ten out of twelve American species of Equisetum are found in Europe. — lb. p. 81. 



t " We often regard a different habitation as a proof of a difference in species, 

 while it may be the cause of the variation." — Gray's Turton, p. 240. Sciylegel, a 

 German herpetologist, after stating his belief that the American Rana halecina and 

 sylvatica are identical with European species, remarks that it is "d"un intcret infini- 

 ment plus puissant, de savoir que nos deux especes de grenouilles communes d'Eu- 

 rope sont repandues a pen pres sur le meme degrc de latitude dans tout I'hemisphere 

 boreal, et quelles presenlent dans des contrees aussi distantes et de nature diverse, 

 comme le sont I'Europe, le nord de I'Afrique, TAsie temperee, le Yapon et I'Ameri- 

 que septentrionale dcs differences si pen notables, que les naturalistes auront de la 

 peine k les signaler, et qu'ils ne viendront pas a bout d'en enumerer de constantes." 

 — Revue Zool., 1838, p. 321. 



