76 INTRODUCTION. 



identity of external form and of organization, have 

 been recorded as different animals. As every rational 

 basis for the distinction of living beings is thus taken 

 away, the advocates of this theory have been driven 

 to seek extra-zoological foundations on which to es- 

 tabhsh it. In one instance the tone of voice has been 

 considered sufficient to separate the well-known bird 

 Corvus Corone of the United States from the same bird 

 in Great Britain. In a zoological family intimately 

 allied to that of which we treat, to wit, the air-breath- 

 ing fresh-water mollusks, or Lim7ieadce, much difficulty 

 has arisen from this source. In this family, both ge- 

 neric and specific forms seem to be almost entirely 

 independent of climatal and other common influences, 

 and the amount of variation among the species is so 

 small that many forms, occurring on both continents, 

 present an uniformity of characters, that would cause 

 them to be considered identical, if confined to one 

 continent only. Hence different names have been im- 

 posed on what we cannot but consider to be the same 

 thing. The terrestrial genus Suceinea, bearing in many 

 respects a strong resemblance to the Limneadoe, is, like 

 them, everywhere very uniform in its external charac- 

 ters, and it is almost impossible to define from them 

 alone, in what our species differ from those of Europe. 

 The question of the identity of these closely allied spe- 

 cies must eventually be decided by their anatomy, but 

 in the mean time we believe it to be perfectly safe to 

 adopt this axiom, that species, whencesoever derived, 



