106 THE ORIGIN OF SPECIES 



BO that, looking to later times, we may believe that the 

 production of uew forms has caused the extinction of 

 about the same number of old forms. 



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 devel- 

 oped 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 hap- 

 pened 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 extermina 

 tion. If many allied forms be developed from the suc- 

 cessful 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 belonging 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 competition. For instance, some species of Tri- 

 gonia, a great genus of shells in the secondary formations, 

 survive in the Australian seas; and a few members of \i 

 the great and almost extinct group of Ganoid fishes still 



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