Translated by Philip H. Nicklin. 115 
I shall cite as examples of this mode of judging, the entirely 
peculiar system of coloring of the Unio irroratus and cylindricus ; 
the unique form of the appendices in the Unio spinosus and in 
the Symphynota bialata (Lea, 1st volume ;) the peculiar form of 
the tubercles in the Unio lacrymosus and apiculatus, &c. 
By combining all these means of exact judgment on the one 
part, and of logical induction on the other, I am satisfied that 
we shall recognize in the group of the Naiades, as experience has 
taught us in others, the truth of that observation, whose expres- 
sion is recorded in the maxim, Natura non facit saltum, viz. that 
exterior unlikeness does not exclude autonomy, and vice versd ; 
or, in other words, that two beings inseparably connected by their 
essential characters, are often much more unlike in their external 
appearance, than two beings entirely different in essential charac- 
ters. 'The demonstration of this proposition is found by the zool- 
ogist, in the races, so abounding in varieties, of the dog, the ox, 
the cock, of man himself compared with the real species but little 
removed, of the quadrumana, the rodentia, the motacille, the 
accipitres, etc. The botanist finds it, in comparing the races so 
prodigiously varied of the pears, the roses, the carnations, of the 
cereal plants and ‘pot-herbs, with the true species, although differ- 
~~ 
ing little in appearance, of the genera Ranunculus, Galium, Ar- 
‘are, Bromus, etc. 'The mineralogist and the 
‘every one knows, at every step of their inves- 
Such, gentlemen, are the principles, upon which, (in default of 
absolute certainty, perhaps not attainable,) I would reform the de- 
marcation of the species of the family of the Naiades. And I 
must say, that no philosopher seems to me at the same time more 
able, and better situated to accomplish it than Mr. Lea himself. 
But has the time arrived in which he can thus occupy himself 
usefully? I think not yet. Possessing the richest of special 
collections, Mr. Lea has not yet, however, had the opportunity of 
inspecting al/ the nominal species. America, where he lives, is 
evidently the privileged country of the Uniones and’ Anodonta, 
considered with respect to the number of species; but America 
has not yet been so entirely explored in this point of view, as not 
to leave a great number of links wanting in that chain, (which 
should be unbroken if possible,) upon which the reformer should 
mark his divisions. It would probably be better, still to'loose the 
