110 Des Moulins on the genera Unio and Anodonta. 



mit of the ladder whose descending steps degrade him to a frog ; 

 and by a further descent even to a monad : by the other, he is 

 placed on the lowest rung of the sublime ladder of created intel- 

 ligences. But, let us speak to our senses : there is a point, in the 

 hundred compartments of which I first spoke, where the homo 

 sapiens finishes, a point where the brute begins ; there is the 

 impracticable leap, the gulf without a bridge, which separates 

 the immaterial from the physical creation : there is the immense, 

 the impenetrable abyss, which the hand of the Almighty has 

 left between the Apollo and the frog. 



It may seem that these reflections are too high, or too abstruse 

 if you will, to serve as the preamble to a critical examination of 

 certain new species of shells. I do not think so, gentlemen ; 

 we can only judge piecemeal ; and when the subject is intellec- 

 tual, the pieces are principles. I thought a full explanation ne- 

 cessary, that you might the better appreciate the elements of the 

 discussion which I propose to submit to your consideration. 

 From what I have stated, I now proceed to draw the single rule 

 that will serve as the basis of my labors. 



Between the first and the last of our hundred compartments, 

 there is a point where the manenAs, where the brute begins; a 

 point at which, notwithstanding the imperceptible deterioration 

 of external form, there is a change of nature, a change of mate- 

 rial classification, a change of order, a change of class, a change 

 of family, a change of genus, a change of species. This consid- 

 eration is enough for my purpose ; without illustrations, its an- 

 nouncement is here sufficient. 



But, gentlemen, the most eminent zoologist now living in 

 France, (M. de Blainville,) stated twelve or fifteen years ago, 

 that in considering the embarrassing variety of forms then known 

 in the genera Unio and Anodonta, he felt the necessity of enqui- 

 ring, whether, if the two genera* were joined again into one, as 

 their comparative anatomy requires, that one should present to 

 the classifying naturalist, an almost endless series of really dis- 

 tinct specie^, or only one species infinitely variable in its forms. 



* In March, 1829, the translator expressed the opinion, " that the seven genera, 

 now referred to the family of JVaiades, are founded in artificial distinctions, and 

 not in nature ; and that in fact the family contains but one genus." — Trans, of the 

 Amer. Phil. Soc, Vol. S,p. 398. 



