Chap. X. EXTINCTION. 099 



other varieties and species, and so on, like tlie branching of a 

 great tree from a single stem, till the group becomes large. 



On Extinction. 



We have as yet spoken only incidentally of the disappear- 

 ance of species and of groups of species. On the theory of 

 natural selection the extinction of old forms and the produc- 

 tion of ne\v and improved forms are intimately connected to- 

 gether. The old notion of all tlie inhabitants of the earth hav- 

 ing been swept away by catastrophes at successive periods is 

 very generally given up, even by tliose geologists, as Elie de 

 Beaumont, ]\Iurcliison, 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 disap- 

 pear, one after another, first from one sj^t, then from another, 

 and finally from tlie world. In some few cases, however, as by 

 the breaking of an isthmus and the conseciuent irrujition of a 

 multitude of new inhabitants into an adjoining sea, or by the 

 final subsidence 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 pres- 

 ent day ; some have disappeared before the close of the paleo- 

 zoic period. No fixed law seems to determine the length of 

 time during which any single species or any single genus en- 

 dures. There is reason to believe that tlic extinction of a 

 whole group of species is generally a slower process than their 

 production : if their appearance and disappearance be repre- 

 sented, as before, by a vertical line of varying tliickness, tlie 

 line is found to taper more gradually at its ujiper end, wliich 

 marks tlie 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 extermina- 

 tion of whole groups, as of ammonites toward the close of the 

 secondary periofl, has been wonderfully sudden. 



The extinction of species has been involved in the most 

 gratuitous mystery. Some authors have even supposed that, 

 as the individual has a definite length of life, so liav<; species 

 a definite duration. No one can have marvelled more than I 

 have done at the extinction of species. "When I found in La 

 Plata the tooth of a horse embedded witli the remains of Mas- 



