288 ON THE ORIGIN OF SPECIES 



The competition will generally be most severe, as 

 formerly explained and illustrated by examples, between 

 the forms which are most like each other in all respects. 

 Hence the improved and modified descendants of a 

 species will generally cause the extermination of the 

 parent -species ; and if many new forms have been 

 developed 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, belong- 

 ing to the same family. But it must often have happened 

 that a new species belonging to some one group will have 

 seized on the place occupied by a species belonging to 

 a distinct group, and thus caused its extermination ; 

 and if many allied forms be developed from the success- 

 ful intruder, many will have to yield their places ; and 

 it will generally be allied forms, which will suffer 

 from some inherited inferiority in common. But 

 whether it be species belonging to the same or to a 

 distinct class, which yield their places to other species 

 which have been modified and improved, a few of the 

 sufferers may often long be preserved, from being 

 fitted to some peculiar line of life, or from inhabiting 

 some distant and isolated station, where they have 

 escaped severe competition. For instance, a single 

 species of Trigonia, a great genus of shells in the 

 secondary formations, survives 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 inter- 

 vals there may have been much slow extermination. 

 Moreover, when by sudden immigration or by unusually 



