102 THE ORIGIN OF SPECIES 



theory of natural selection, the extinction of old forme 

 and the production of new and improved forms are inti- 

 mately connected together. The old notion of all the 

 inhabitants of the earth having been swept away by 

 catastrophes at successive periods is very generally given 

 up, even by those geologists, as Elie de Beaumont, 

 Murchison, Barrande, etc., whose general views would 

 naturally lead them to this conclusion. On the contrary, 

 we have every reason to believe, from, the study of the 

 tertiary formations, that species and groups of species 

 gradually disappear, one after another, first from one 

 spot, then from another, and finally from the world. In 

 some few cases, however, as by the breaking of an isth- 

 mus and the consequent irruption of a multitude of new 

 inhabitants into an adjoining sea, or by the final subsi- 

 dence of an island, the process of extinction may have 

 been rapid. Both single species and whole groups of 

 species last for very unequal periods; some groups, 

 as we have seen, have endured from the earliest known 

 dawn of life to the present day; some have disappeared 

 before the close of the paleozoic period. No fixed law 

 seems to determine the length of time during which any 

 single species or any single genus endures. There is 

 reason to believe that the extinction of a whole group 

 of species is generally a slower process than their pro- 

 duction: if their appearance and disappearance be repre- 

 sented, as before, by a vertical line of varying thickness, 

 the line is found to taper more gradually at its upper 

 end, which marks this progress of extermination, than at 

 its lower end, which marks the first appearance and the 

 early increase in number of the species. In some cases, 

 however, the extermination of whole groups, as of am- 



