398 ON LAMARCK S FAMILY 



ral Sciences; where, after describing Alasmodonta ambigua, 

 he says, " It forms a link between the genera Alasmodonta 

 and Anodonta. When young the primary teeth are obvious, 

 but when the shell arrives at the full groivth, the teeth are 

 obsolete, and in some instances not at all visible." 



From this it appears that some shells of the family of 

 Naiades, at different ages, assume different appearances in 

 those parts which naturalists have fixed upon for the distinc- 

 tive characteristics of different genera. How cautious should 

 we be not to suffer our ingenuity to run before the uner- 

 ring indications of Nature ! 



Mr Barnes, to whose suggestions great deference is due, in 

 speaking of the almost infinite and nearly indistinguishable 

 varieties of the genus Unio, observes, " that the thought had 

 frequently struck him, that, properly speaking, there is but 

 one species of the whole genus, and perhaps of the ivhole fa- 

 mily. See Silliman's Journal, vol. vi. p. 115; the whole pa- 

 ragraph is well worth attention. 



This opinion goes rather too far, but is still strongly corro- 

 borative of the conclusion at which it is wished to arrive, 

 namely, that the seven genera, now referred to the family of 

 Naiades, are founded in artificial distinctions, and not in Na- 

 ture ; and that in fact the whole family contains but one ge- 

 nus, which was originally established by Bruguieres, and 

 should be called Unio. In that case, the present genera 

 might be considered as so many species, and the present spe- 

 cies as so many varieties : or, the genus Unio might be divi- 

 ded into subgenera and species. 



It often happens with young naturalists that the thirst of 

 fame is greater than the desire of knowledge, which has 

 caused the books to be swelled with genera and species that 

 exist only in imagination ; even the illustrious Lamarck has 

 erred in this respect ; and in the genus Unio has probably de- 

 scribed as different species five or six varieties of Say's purpu- 

 reas. 



Linnaeus, whose comprehensive mind seemed to scan the 

 universe at a glance, was governed by the severest simplicity 



