Ch. XIV.] 



IN THE ANIMATE WORLD. 



311 



races in plants and animals, and new forms are continually 

 supplanting others which had endured for ages. But natural 

 history has been successfully cultivated for so short a period, 



exam 



same 



of absolute, extirpation of species can as yet be proved, and 

 these only where the interference of man has been conspi- 

 cuous. It will nevertheless appear evident, from the facts and 

 arguments detailed in the Third Book, in the chapters which 

 treat of the geographical distribution of species in the next 

 volume, that man is not the only exterminating agent ; and 

 that, independently of his intervention, the annihilation of 

 species is promoted by the multiplication and gradual diffusion 

 of every animal or plant. It will also appear, that every 

 alteration in the physical geography and climate of the globe 



tendency. If we proceed still 

 farther, and enquire whether new species are substituted from 

 time to time for those which die out, and whether there are 

 certain laws appointed by the Author of Nature to regulate 

 such new creations, we find that the period of human obser- 

 vation is as yet too short to afford data for determining so 

 difficult a question. All that can be done is to show, that the 

 successive introduction of new species may be a constant part 

 of the economy of the terrestrial system, without our having 

 any right to expect that we should be in possession of direct 

 proof of the fact. To enable the reader to appreciate the 

 radual 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 



mor 



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 Eocene, or oldest subdivision, some few of the 



testacea belong to existing species, although almost all of 



them, and apparently all the associated vertebrata, are now 

 extinct. These Eocene strata are succeeded by a great num- 

 ber of more modern deposits, which depart gradually in the 

 character of their fossils from the Eocene type, and approach 

 more and more to that of the living creation. In the present 



