Chap. XL] EXTINCTION. 99 



from any one species, the nearest allies of that species, 

 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 wi^l generally be 

 the alhed forms, which will suffer from some inherited 

 inferiority in common. But whether it be species be- 

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

 yielded their places to other modified and improved spe- 

 cies, 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 competition. For 

 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 extermina- 

 tion 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 extermination. 

 Moreover, when, by sudden immigration or by unusu- 

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