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 phenomena. 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--"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 perfectly 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 influence 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 geological speculations grounded on the first opinion, be ultimately abandoned. The geographical distribution of organic remains, is an exceedingly curious inquiry. If accurately pursued, without reference to any preconceived theory, it will no doubt furnish much information as to the comparative ages of the different strata which compose the external crust of our planet--for that these strata were deposited or formed at periods of time more or less remote from each other, every one knows, to be a generally admitted _geological fact_. The occurrence of similar fossils in districts of country remotely situated from each other, certainly presents a phenomenon highly interesting to the speculative naturalist, and apparently indicates that the same powerful and general causes must have concurred to produce these isomorphous depositions. No fossils have contributed more to this kind of information, than those of shells, and as the mineralized species could not be satisfactorily studied, except by accurately comparing them with those which now inhabit our seas and continents; the search for shells, has become, from a simple amusement, the study of scientific men--or, as a writer remarks, "it was only after the period when it was perceived that geology and ancient zoology were destined to be enlightened by their fossil remains, that this research passed from the hands of amateurs into those of naturalists."[5]