292 THE GEOLOGICAL SUCCESSION [Chap. XI. 



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

 did not become in some degree modified and improved, would bo 

 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 accu- 

 mulated 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, fhould 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 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 

 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 in some 

 degree different, and the newly-formed variety would probably 

 inherit from its progenitor some characteristic 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 

 degree. A group, when it has once disappeared, never reappears ; 

 that is, its existence, as long as it lasts, _ is continuous. I am 

 aware that there are some apparent exceptions to this rule, but the 

 exceptions are surprisingly few, so few that E. Forbes, Pictet, and 



