V 



584 ANCIENT AND EXISTING DISTRIBUTION OF SPECIES CONTRASTED. 



But it has been said, that a species in these old rocks has been often decided by a 

 mere cast, grounds upon which no conchologist can pretend to set up real distinctions. 

 Now to this objection I would first reply, that sharply defined casts may sometimes 

 suffice to prove the identity of forms to be compared, and secondly, that it is by no 

 means on such evidences alone, that my conclusions have been drawn ; for in number- 

 less instances the fossils of the Silurian System preserve their shelly covering as perfectly 

 as the testacea even of the youngest deposits. 



On this point, however, of specific identity (particularly in the case of very distant 

 deposits) let us not insist too strongly. It is enough to state, that on comparing the 

 fossils from different localities of each Silurian formation we find the closest agreement ; 

 certain forms being found in one formation, certain other forms in another ; and by 

 this means we have been able to determine the age of the rocks, and to verify the just- 

 ness of the division by order of superposition. 



Extending this mode of investigation to fossils obtained from other parts of the world , 

 and comparing casts with casts, an identity of form as well as of muscular and other 

 impressions will be at once admitted. It is on these grounds we assert, that while the 

 Silurian System was in the course of deposition, similar mollusks, crustaceans and corals, 

 lived in very distant parts of the globe, and where climates of very different tempera- 

 ture now prevail. 



In the meantime we may appeal to our records in the rocks, and if each succeeding 

 system is found to contain different species of animals, there can be no difficulty in de- 

 termining, whether their former geographical distribution was or was not similar to that 

 which now prevails. 



Examining the strata with this view, we find that in the ascending geological series 

 the quantity of species increases considerably as we approach the younger deposits, and 

 that in proportion to this increase, their geographical distribution harmonizes more and 

 more with that of existing nature. For example, in the older Tertiary strata (Eocene), 

 there are scarcely any examples of shells identical, being found in opposite hemispheres, 

 and in the younger Tertiary (Pliocene) absolutely none ; though in these deposits the 

 number of species is at least tenfold greater than in the ancient rocks under consideration. 



Now if this position be admitted, it is manifest that it cannot be explained by refer- 

 ence to the present distribution of animal life. For not only has each latitude, in our 

 days, its different products, but even in distant parts of the same latitude (and in the 

 same hemisphere), the amount of variation of species is often very great. 



Again^ reverting for an instant to our argument in the last chapter, we must recol- 

 lect, that it does not merely depend upon the wide area over which the sediment con- 

 taining these fossils was spread, but also upon the fact, that these ancient rocks are of 

 vastly greater thickness than any of the succeeding deposits, and must therefore have 

 occupied a much longer period during their formation. 



Hence it is that the geologist, whose science consists in unfolding the records of lost 



