GEOLOGICAL SUCCESSION OF ORGANIC BEINGS 99 



When many of the inhabitants of any area have become 

 modified and improved, we can understand, on the prin- 

 ciple of competition, and from the ail-important relations 

 of organism to organism in the struggle for 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 enough intervals of time, become modi- 

 fied, 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; con- 

 sequently the amount of organic change exhibited by the 

 fossils imbedded 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 condi- 

 tions 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 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 



