Baron A. Humboldt on Petrifactions or 
limestone, among reputed fluviatile fishes, in the bituminous 
slate of the Alpine limestone ? Do identical species of petrifac- 
tions or fossils occur in the same formations in different points of 
the globe ? Are they capable of furnishing zoological characters 
for distinguishing the different formations ? Or, should it not 
rather be admitted, that species which the zoologist is entitled to- 
consider as identical, following the methods usually adopted, 
penetrate across several formations ; that they are even seen in 
those which are not in immediate contact? Should not the 
zoological characters be taken both from the total absence of 
certain species, and from their relative frequency or predomi- 
nance. , as well as from their constant association with a certain 
Humber of other species? Is it right to divide a formation, 
whose unity has been determined from relations of position, and 
from the identity of the beds, which are equally interposed be- 
tween upper and lower strata, merely because the first of these 
strata contain fresh-water shells, while the last contain marine 
ones ? Is the total absence of organic bodies in certain masses 
of secondary and tertiary formations, a sufficient reason for con- 
sidering these masses as particular formations, if other geognos- 
tical relations do not justify this separation ? 
Some of these problems engaged the attention of geologists at an 
early period. Lister had already maintained, upwards of 150 years 
ago, tli at each rock was characterized by different fossil shells, — 
{Philosophical Trans., No. 76, p. 2288.) With the view of pro- 
ving that the shells of our seas and lakes are specifically different 
from fossil shells (lapides sui. generis ), he affirms, 44 that the 
latter, for example, those of the Northamptonshire quarries, 
have all the characters of our marices , telling, and trochi ; but 
that naturalists, who are not accustomed to content themselves 
with a vague and general perception of things, will find the fos- 
sil shells specifically different from those of the present world.'’ 
Nearly at the same period, Nicolas S ten on, {Be solido intra 
solidum contenio; , 1668, p. 2, 17, 28, 68, 69, fig. 20, 25,) 
first distinguished 44 the (primitive) rocks anterior to the 
existence of plants and animals upon the globe, and conse- 
quently never containing organic remains, and the (secondary) 
ro;ks superimposed upon the first, and abounding with these 
remains (turbidi mavis scdimenta sibi invicem imposita ).” 
