2 SYNOPSIS OF 



which can be considered absolute. For although in a natural 

 system, all other combinations, whether genera, tribes, orders, 

 or by whatever name they may be known, comprehend spe- 

 cies agreeing much more with each other than with anything 

 else, and having a positive general resemblance in the majority 

 of their features, yet no fixed limits can be assigned to any of 

 them ; on the contrary, they pass, by means of various inter- 

 mediate species, into the other genera, tribes, orders, &c., to 

 which they are most nearly allied. For this reason, viz., that 

 no fixed limits can be assigned to orders, genera, &c., we find 

 the ideas about them fluctuating with the degree of our know- 

 ledge ; which is the true cause of those changes in the limits 

 of genera, &c., which persons unacquainted with the subject 

 are apt to consider arbitrary ; but which, in skilful hands, are 

 dependent upon a progressive advance in the knowledge of 

 science." 



Blainville, in his " Manuel de Malacologie," divides the 

 Naiades (his Sub-mytilacea) into Anodonta and Unio, but 

 thinks that species will be found which will make these to be 

 united.* 



Sowerby says, " the difficulty of ascertaining to which genus 

 of Lamarckian Naiades certain species belong, arises from the 

 very general similarity of form," &c. ; " in fact, an examina- 

 tion of a sufficient number of species will prove that no de- 

 pendence can be placed upon the characters by which authors 

 usually attempt to discriminate between these genera, and that 

 the transition from one to another is so gradual in some in- 

 stances, and so strongly marked in others, that it is not sur- 



* See page 540. 



