Chap. X. GEOLOGICAL SUCCESSION. 315 



will be liable to be exterminated. Heuce we can 

 see why all the species in the same region do at last, 

 if we look to wide enough intervals of time, become 

 modified ; for those which do not change Avill become 

 extinct. 



In members of the same class the average amount of 

 change, during long and equal periods of time, may, 

 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 con- 

 secutive 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 drania. 



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 innume- 

 rable 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 cer- 

 tainly inherit different characters from their distinct 

 progenitors. 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 

 present 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 supplanted and 



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