Chap. XI.] EXTINCTION. 99 



i.e. the species of the same genus, will be the most liable 

 to extermination. Thus, as I believe, a number of new 

 species descended from one species, that is a new genus, 

 comes to supplant an old genus, belonging to the same 

 family. But it must often have happened that a new 

 species belonging to some one group has seized on the 

 place occupied by a species belonging to a distinct group, 

 and thus have caused its extermination. If many allied 

 forms be developed from the successful intruder, many 

 will have to yield their places ; and it will generally be 

 the allied forms, which will suffer from some inherited 

 inferiority in common. But whether it be species 

 beloncnnsf to the same or to a distinct class, which have 

 yielded their places to other modified and improved 

 species, a few of the sufferers may often be preserved 

 for a long time, from being fitted to some peculiar line 

 of life, or from inhabiting some distant and isolated 

 station, where they will have escaped severe competi- 

 tion. Tor instance, some species of Trigonia, a great 

 genus of shells in the secondary formations, survive in 

 the Australian seas ; and a few members of the great and 

 almost extinct group of Ganoid fishes still inhabit our 

 fresh waters. Therefore the utter extinction of a group 

 is generally, as we have seen, a slower process than its 

 production. 



With respect to the apparently sudden extermination 

 of whole families or orders, as of Trilobites at the close 

 of the palaeozoic period and of Ammonites at the close of 

 the secondary period, we must remember what has been 

 already said on the probable wide intervals of time 

 between our consecutive formations ; and in these 

 intervals there may have been much slow extermina- 

 tion. Moreover, when, by sudden immigration or by 



