Theory of Yarieiies, 



5887 



bivalve of if it be removed from the stone or shell to which it adheres, 

 since the under valve is a mere layer of shelly matter insaperably 

 united to the substratum) is connnon in the deeper parts in and out- 

 side of Lamlash Bay. I have also taken it, though less frequently, 

 to the west of Cunibrae, and in other parts. The largest examples 

 from the Clyde do not exceed half an inch in diameter. 



Errata in the previous Part. — Page 5711, line 5, for " Apatinida" read " Anati- 

 nida"; line 22, for Amphidesma convexa" read Amphidesma convex um" Page 

 5712, line 15, for " pellucidens" read "pellueidus." Page 5713, line 31, dele 

 " I have " to " appears scarce '' ; line 34, for " compressa " read " compressum." 



Alfred Merle Norman. 



Kibworth, Leicestershire, 

 October 28, 1857. 



(To be continued). 



Note on the Theory of Permanent and Geographical Varieties. 

 By Alfred R. Walllace, Esq. 



As this subject is now attracting much attention among naturalists, 

 and particularly among entomologists, I venture to offer the following 

 observations, which, without advocating either side of the question, 

 are intended to point out a difficulty, or rather a dilemma, its advo- 

 cates do not appear to have perceived. 



The adoption of permanent and geographical varieties has this 

 disadvantage, that it leaves the question " What is a species f'' more 

 indeterminate than ever; for if permanent characters do not constitute 

 one when those characters are minute, then a species differs from a 

 variety in degree only, not in nature, and no two persons will agree as 

 to the amount of difference necessary to constitute the one, or the 

 amount of resemblance which must exist to form the other. The line 

 that separates them will become so fine that it will be exceedingly- 

 difficult to prove its existence. If, however, the two things are of es- 

 sentially distinct natures, we must seek a qualitative not a quantitative 

 character to define them. This may be done by considering the per- 

 manence, not the amount, of the variation from its nearest allies, to 

 constitute the specific character, and in like manner the instability, 

 not the smaller quantity, of variation to mark the variety. In this 

 way you define the two things by a difference in their nature ; by the 

 other, you assert that they are of exactly the same nature, and differ 

 only in degree. 



