92 THE GEOLOGICAL SUCCESSION [Chap. XL 



ism to organism in the struggle for life, that any form 

 which did not become in some degree modified and im- 

 proved, would be liable to extermination. Hence we see 

 why all the species in the same region do at last, if we 

 look to long enough intervals of time, become 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 accumu- 

 lated at wide and irregularly intermittent intervals of 

 time; consequently the amount of organic change ex- 

 hibited by the fossils embedded in consecutive forma- 

 tions 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 

 instances) to fill the place of another species in the econ- 

 omy 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 organ- 

 isms 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 



