UNIFORMITY OF CHANGE 425 



now become fixed and stationary, we discover that, on the 

 contrary, it is in a state of continual flux that there are 

 many causes in action which tend to the extinction of species, 

 and which are conclusive against the doctrine of their un- 

 limited durability. 



There are also causes which give rise to new varieties and 

 races in plants and animals, and new forms are continually 

 supplanting others which had endured for ages. But natural 

 history has been sucessfully cultivated for so short a period, 

 that a few examples only of local, and perhaps but one or 

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

 and these only where the interference of man has been con- 

 spicuous. It will nevertheless appear evident, from the facts 

 and arguments detailed 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, inde- 

 pendently 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 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 Eocene, or oldest subdivisions, 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 



