Masterpieces of Science 



geography and climate of the globe cannot fail 

 to have the same tendency. If we proceed still 

 farther, and enquire whether new species are 

 substituted from time to time for those which die 

 out, we find that the successive introduction of 

 new forms appears to have been a constant part 

 of the economy of the terrestrial system, and if 

 we have no direct proof of the fact it is because 

 the changes take place so slowly as not to come 

 within the period of exact scientific observation. 

 To enable the reader to appreciate the gradual 

 manner in which a passage may have taken place 

 from an extinct fauna to that now living, I shall 

 say a few words on the fossils of successive 

 Tertiary periods. When we trace the series of 

 formations from the more ancient to the more 

 modern, it is in these Tertiary deposits that we 

 first meet with assemblages of organic remains 

 having a near analogy to the fauna of certain 

 parts of the globe in our own time. In the Eo- 

 cene, or oldest subdivisions some few of the 

 testacea [animals having hard shells] belong to 

 existing species, although almost all of them, 

 and apparently all the associated vertebrata, 

 are now extinct. These Eocene strata are suc- 

 ceeded by a great number of more modern de- 

 posits, which depart gradually in the character 

 of their fossils from the Eocene type, and ap- 

 proach more and more to that of the living 

 creation. In the present state of science, it is 

 chiefly by the aid of shells that we are enabled to 

 arrive at these results, for of all classes the tes- 

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