324 THE GEOLOGICAL SUCCESSION" 



pared with marine and lower productions, by the more com- 

 plex relations of the higher beings to their organic and 

 inorganic conditions of life, as explained in a former chapter. 

 When many of the inhabitants of any area have become 

 modified and improved, we can understand, on the principle of 

 competition, and from the all-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 in- 

 tervals 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 sedi- 

 ment 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 conditions 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 would vary in a different man- 

 ner. 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 parent forms are generally 

 supplanted and exterminated by their improved offspring, it 

 is incredible that a fantail, identical with the existing breed, 

 could be raised from any other species of pigeon, or even 

 from any other well-established race of the domestic pigeon, 

 for the successive variations would almost certainly be. ia 



