CONTRIBUTIONS TO CONCHOLOGY. 



Ho. 11. 



Hints on the Geographical Distribution of Animals, 

 with Special Reference to the Mollusca. By C. B. Adams. 

 October, 1852. 



Areas of Species. 



1. Each species occupies one geographical area only. 



2. Species * introduced ' by human agency from one area into 

 another are obvious exceptions. 



3. The literature of Zoology, especially of Conchology, 

 conflicts with the first proposition (§ 1) in a great number of 

 examples, chiefly on account of the common practice of pub- 

 lishing statements of habitats, without thorough scrutiny into 

 their authenticity.* 



4. Inasmuch as natural types are of all grades of value (v. p. 

 191), so that some are of a little less and others of a little more 

 than the average value or importance of a species, the difficulty 

 which hence arises in the discrimination of species must cause 

 some apparent or doubtful exceptions to our first proposition. 

 The differences between some types, which inhabit distinct 

 areas, is slight : and other types exist, between which differ- 

 ences have not yet been detected, although ihey may actually 

 exist.f If such difterences prove to be absolutely constant, the 

 types will generally be regarded as good species. But if other- 



* We have elsewhere proposed to distinguish original from hearsay testimony, 

 by an exclamation mark. (!) 



f Of sucli types Dr. Gould "admits it as an axiom in Zoology," that they 

 " should be assumed as dififerent, until their identity can be proved." — Introd 

 Moll. Expl. Expcd. p. XL 



No. 11.— 1 



