292 SHELLS AND SHELL-FISH. PART I. 



manner, if he took all the winged or connate shells, 

 and, holding as trivial their other characters, proceeded 

 to place them in a separate genus, he would be doing 

 the same thing, he would he confounding objects 

 which merely represented each other analogically, or, as 

 it were, allegorically, under the belief that they were 

 bond fide connected by affinity, and followed each other 

 in the scale of nature. 



(272.) But if all this be admitted, the difficulty of 

 separating species from varieties still remains : a species, 

 it has been said, is the only absolute division in nature ; 

 and did we look to the vertebrated, and even the annu- 

 lose, animals, we might be tempted to adopt this as an 

 axiom ; but in regard to the present family, at least, it 

 cannot be maintained, for it receives a direct contradic- 

 tion. The mode of generation in the whole of the 

 Dithyra precludes the theory of promiscuous intercourse, 

 and the consequent production of hybrids.* The vari- 

 ations, therefore, are spontaneous, and lie beyond our in- 

 vestigation. We have seen, however, in the case of the 

 Unio Mytil'iodes, that while Nature appears to disregard 

 all her usual bounds, and to indulge in almost endless 

 diversity, she nevertheless strictly confines herself to the 

 same plan she has pursued in all other groups. Her vari- 

 ations are upon the same system as that which pervades 

 the animal world. She creates, in short, a type, all the 

 variations of which have a reference to, and often the 

 very aspect of, those particular forms which mark the 

 primary divisions of the whole family. This theory, 

 borne out in almost every group here laid before the 

 reader, will materially, if not absolutely, guide us in 

 determining the limits of species, and consequently of 

 varieties. We trust the rising school of American ma- 

 lacologists will confirm this by a renewed investigation of 

 the inexhaustible profusion of Unionida with which 

 their noble rivers abound. Having seen but very few of 



* On all anatomical facts, we look to the illustrious Cuvier as a para- 

 mount authority ; he expressly says all the Dithyra are hermaphrodites. 

 Mr. Lea, however, mentions male and female shell-fish of the same spe- 

 cies. Is there no error in this ? 



