Chap. X. 
GEOLOGICAL SUCCESSION. 
315 
will be liable to be exterminated. Hence 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 will 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 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 ofispring 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 
exterminated by its improved offspring, it is quite in- 
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