11 



found in the rocks of North America^ 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. 



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 

 geology u 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. 

 The 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 



