iT. * 
found in the rocks of North Americas we leave to 
other and abler hands the more difficult and interest- 
ing task of determining with precision the connex- 
ion which may exist between these organic reliques, 
and the relative ages of the strata in which they are 
found. oie 
It is supposed, indeed, that a sufficient number of 
well characterized species have not yet been collected 
and accurately described, to throw any certain and 
clear light on otherwise doubtful geological pheno- 
mena. What has been remarked by De Candolle, 
with regard to botanical geography, is perhaps true 
of these fossils as to solving the difficult problems of 
seology—“ Let us not forget,” says he, “ that this 
science can only be commenced when the study of 
species has been sufficiently advanced to furnish us 
with numerous and well authenticated facts.”’ 
We are well aware of the difficulty of settling the 
line which ought to divide species. Individuals per- _ 
fectly identical in all their parts, are rarely, if ever 
seen; though a general resemblance may be easily 
traced. Among fossils, just discriminations of this 
kind are more delicate, than in recent specimens. 
fhe hand of time, accidental causes, and the influ- 
ence of atmospheric changes often produce such 
characters as to render the determination of fossil 
species an exceedingly difficult task. We have no 
doubt, therefore, that a few of our Trilobites, which 
are now considered as perfectly identical with some 
found in Europe, will upon fuller examination, be 
discovered to be dissimilar, and of course certain 
