GEOLOGICAL SUCCESSION 285 



slowly converted into species, which in their turn pro- 

 duce by equally slow steps other species, and so on, like 

 the branching of a great tree from a single stem, till 

 the group becomes large. 



On Extinction. — We have as yet spoken only inci- 

 dentally of the disappearance of species and of groups 

 of species. On the theory of natural selection the ex- 

 tinction of old forms and the production of new and im- 

 proved forms are intimately connected together. The 

 old notion of all the inhabitants of the earth having 

 been swept away at successive periods by catastrophes, 

 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 con- 

 clusion. 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. Both single species and 

 whole groups of species last for very unequal periods ; 

 some groups, as we have seen, having endured from the 

 earliest known dawn of life to the present day ; some 

 having disappeared before the close of the palaeozoic 

 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 complete extinction of the species of a group is 

 generally a slower process than their production : if the 

 appearance and disappearance of a group of species be 

 represented, as before, by a vertical line of varying 

 thickness, the line is found to taper more gradually at 

 its upper end, which marks the progress of extermina- 

 tion, than at its lower end, which marks the first 

 appearance and increase in numbers of the species. In 

 some cases, however, the extermination of whole groups 

 of beings, as of ammonites towards the close of the 

 secondary period, has been wonderfully sudden. 



The whole subject of the extinction of species has 

 Heen involved in the most gratuitous mystery. Some 



