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Translated by Philip H. Nicklin. lil 
I do not think that M. de Blainville, seeing the ict materials 
since published by Mr. Isaac Lea, would fiow feel’a similar hesi- 
tancy. It may be supposed, that in a series of specimens more 
or less numerous, selected with care from thé different domestic 
and foreign forms, the passage may be almost insensible from»the 
immensely large cardinal teeth of the Unio 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 Unio circulus to the linear form of the Unio 
rectus, exhibiting successively all the modifications of form con- 
tained between these extremes; and I 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 levissimus ; which is so thin that atmos- 
pheric drought makes it split ; or to pases on whose form is 
almost as compressed as that of certain Lutrari@, 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 
exhibits to our admiration. ; 
ut in the 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 num- 
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. 
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