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- 
tablish 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 Limneacke, 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 Succinea, bearing in many 
respects a strong resemblance to the Limneadce, 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, whenccsoever derived. 
