IjMTKODUCTION. 73 



living individuals, he soon perceives that the divisions 

 are much less abrupt than he had supposed. He finds 

 that the changes of type are gradual and almost insensi- 

 ble, and that some of the forms that had appeared to 

 him definite and fixed, blend by degrees with others 

 ■which he had considered to be distinct. He necessarily 

 abandons many of his preconceived views as to spe- 

 cific distuictions, and is obliged to modify all of them 

 more or less by his new experience. Every naturahst 

 should therefore test his opmions by an intimate ac- 

 quaintance "v\ith livmg species existing m their natural 

 conditions. It is true that but few of our naturalists 

 could heretofore comply with these prehminary requisi- 

 tions, and perhaps the most complete collections in the 

 country, public or private, have not been sufiiciently 

 ample to afibrd the means of making so thorough a 

 comparison as we consider to be necessary ; yet the 

 rules we lay down are good ones, and, if observed so 

 far as circumstances will permit, will be of great ser- 

 vice to science. K our naturalists had been guided 

 by them during the short career of American zoology, 

 we should now be free from an amomit of error in re- 

 spect to our own species, which, in the period of twenty- 

 five years, has created a synonymy well nigh as con- 

 fused as that which in Europe has been accumulating 

 more than t^vice that length of time. The excuse for 

 error which formerly existed, in the want of means for 

 learning what is correct, can no longer be received, for 

 the facihties arising from ample collections of shells and 



VOL. I. 9 



