296 EXTINCTION. [Chap. XI. 



maintained by having some advantage over those with which it 

 comes into competition ; and the consequent extinction of the less- 

 favoured forms almost inevitably follows. It is the same with our 

 domestic productions ; when a new and slightly improved variety 

 has been raised, it at first supplants the less improved varieties in 

 the same neighbourhood ; when much improved it is transported 

 far and near, like our short-horn cattle, and takes the place of other 

 breeds in other countries. Thus the appearance of new forms and 

 the disappearance of old forms, both those naturally and those arti- 

 ficially produced, are bound together. In flourishing groups, the 

 number of new specific forms which have been produced within a 

 given time has at some periods probably been greater than the 

 number of the old specific forms which have been exterminated ; 

 but we know that species have not gone on indefinitely increasing, 

 at least during the later geological epochs, so that, looking to later 

 times, we may believe that the production of new forms has caused 

 the extinction of about the same number of old forms. 



The competition will generally be most severe, as formerly ex- 

 plained 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 extermi- 

 nation 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 extermi- 

 nation. 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 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 competi- 

 tion. 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. 



