298 THE ORIGIN OF SPECIES 



habitants of any area have become modified and improved, we 

 ' can understand, on the principle of competition, and from the all- 

 I important relations of organism to organism in the struggle for 

 \ Hie, that any form which did not become in some degree modified 

 Vand improved, would be liable to extermination. Hence, we see 

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

 I 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 ac- 

 cumulated at wide and irregularly intermittent intervals of time; 

 consequently the amount of organic change exhibited by the fos- 

 sils embedded in consecutive formations is not equal. Each forma- 

 tion, 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 offsprmg 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 progeni- 

 tors; 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 dis- 

 tinguishable 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 exter- 

 minated 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 in some degree different, and the newly-formed 

 variety would probably inherit from its progenitor some character- 

 istic differences. 



Groups of species, that is, genera and families, follow the same 

 general rules in their appearance and disappearance as do single 

 species, changing more or less quickly, and in a greater or lesser 



