EXTINCTION. 329 



merly 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 

 ■>ne species, the nearest allies of that species, i. e., the 

 species of the same genus, will be the most liable to exter- 

 mination. Thus, as I believe, a number of new species de- 

 scended 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 

 it« 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 belonging to the same or to a distinct class, 

 which have yielded their places to other modified and im« 

 proved species, a few of the sufferers may often be pre- 

 served 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. There- 

 fore, 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 out 

 consecutive formations ; and in these intervals there may 

 have been much slow extermination. Moreover, when, by 

 sudden immigration or by unusually rapid development, 

 many species of a new group have taken possession of an 

 area, many of the older species will have been exterminated 

 in a correspondingly rapid manner; and the forms which 

 thus yield their places will commonly be allied, for they will 

 partake of the same inferiority in common. 



Thus, as it seems to me, the manner in which single 

 species and whole groups of species become extinct accords 



