GEOLOGICAL SUCCESSION 283 



perhaps, be nearly the same ; but as the accumulation 

 of long-enduring fossiliferous formations depends on 

 great masses of sediment having been deposited on 

 areas whilst subsiding, our formations have been almost 

 necessarily accumulated at wide and irregularly inter- 

 mittent intervals ; consequently the amount of organic 

 change exhibited by the fossils embedded in consecutive 

 formations is not equal. Each formation, on this view, 

 does not mark a new and complete act of creation, but 

 only an occasional scene, taken almost at hazard, in a 

 slowly changing drama. 



We can clearly understand why a species when once 

 lost should never reappear, even if the very same con- 

 ditions of life, organic and inorganic, should recur. 

 For though the offspring of one species might be 

 adapted (and no doubt this has occurred in innumer- 

 able instances) to fill the exact place of another species 

 in the economy of nature, and thus supplant it ; yet 

 the two forms — the old and the new — would not be 

 identically the same ; for both would almost certainly 

 inherit different characters from their distinct pro- 

 genitors. For instance, it is just possible, if our 

 fantail-pigeons were all destroyed, that fanciers, by 

 striving during long ages for the same object, might 

 make a new breed hardly distinguishable from our pre- 

 sent fantail ; but if the parent rock-pigeon were also 

 destroyed, and in nature we have every reason to 

 believe that the parent -form will generally be sup- 

 planted and exterminated by its improved offspring, it 

 is quite incredible that a fantail, identical with the , 

 existing breed, could be raised from any other species 

 of pigeon, or even from the other well-established races 

 of the domestic pigeon, for the newly -formed fantail 

 would be almost sure to inherit from its new progenitor 

 some slight characteristic differences. 



Groups of species, that is, genera and families, follow 

 the same general rules in their appearance and dis- 

 appearance as do single species, changing more or less 

 quickly, and in a greater or lesser degree. A group 

 doe* not reappear after it has once disappeared ; or its 



