Translated by Philip H. Nickliti. Ill 



1 do not think that M. de Blainville, seeing the rich materials 

 since published by Mr. Isaac Lea, would now feel a similar hesi- 

 tancy. It may be supposed, that in a series of specimens more 

 or less numerous, selected with care from the different domestic 

 and foreign forms, the passage may be almost insensible from the 

 immensely large cardinal teeth of the U77io crassissima to the 

 toothless hinge of the Anodonta cygnea. But if a hundred spe- 

 cimens be necessary to pass through all these successive grada- 

 tions ; I dare say another series of different specimens quite as 

 numerous, would be required to pass insensibly from the very 

 round form of the XJnio circulus to the linear form of the Unio 

 rectvs, exhibiting successively all the modifications of form con- 

 tained between these extremes; and 1 warrant that still new se- 

 ries almost as numerous, would be necessary to form a commu- 

 nication between the most solid and stony species of the United 

 States, and the Unio Icevissimvs ; which is so thin that atmos- 

 pheric drought makes it split ; or to connect those whose form is 

 almost as compressed as that of certain Lutrarice, with those 

 whose three diameters are nearly equal. In fine, I do not think 

 that a single series, however numerous, can exhibit at once all 

 these combinations, with all these various conditions of gradation. 

 And since it required ninety eight designs, almost entirely imagi- 

 nary, to make an uninterrupted chain between the Apollo and 

 the frog, what would the number be, when we should find our- 

 selves engaged with nature herself, so rich in resources, so abound- 

 ing in ornament, so inexhaustible in the variety of forms that she 

 exlid^ to our admiration. 



^4 '" ^^^^ same manner, (and even oftener than the ladder of 

 ninety eight steps which separates the Apollo from the frog,) 

 comparative anatomy, (which also offers us a clear scale of per- 

 ceptible gradations,) discovers to us the successive and gradual 

 changes of orders, of classes, of families, of genera and of spe- 

 cies ; and it is enough to cast the eye over a rich collection of 

 Naiades, or over the magnificent figures contained in Mr. Lea's 

 two volumes, to perceive there at the first glance, a certain nimi- 

 ber of typical forms, distinct, and each surrounded by derivative 

 forms, more or less numerous ; which are too closely connected 

 with them to be separated from them, or even to form connecting 

 links between them and other types entirely different. 



