92 THE GEOLOGICAL SUCCESSION [Chap. XL 



life, that any form which did not become in some degree 

 modified and improved, would be liable to extermination. 

 Hence we see why all the species in the same region do 

 at last, if we look to long enongh intervals of time, be- 

 come modified, for otherwise they would 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 enduring formations, rich in fossils, depends on great 

 masses of sediment being deposited on subsiding areas, 

 our formations have been almost necessarily accumulated 

 at wide and irregularly intermittent intervals of time ; 

 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 an ever 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 innumerable in- 

 stances) to fill the 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 progenitors ; and organisms 

 already differing would vary in a different manner. 

 For instance, it is possible, if all our fantail pigeons 

 were destroyed, that fanciers might make a new breed 

 hardly distinguishable from the present breed ; but if 

 the parent rock-pigeon were likewise destroyed, and 

 under nature we have every reason to believe that 



