GEOLOGICAL SUCCESSION 293 



competition with many already existing forms, would be 

 highly favourable, as would be the power of spreading 

 into new territories. A certain amount of isolation, 

 recurring at long intervals of time, would probably be 

 also favourable, as before explained. One quarter of 

 the world may have been most favourable for the pro- 

 duction of new and dominant species on the land, and 

 another for those in the waters of the sea. If two great 

 regions had been for a long period favourably circum- 

 stanced in an equal degree, whenever their inhabitants 

 met, the battle would be prolonged and severe ; and 

 some from one birthplace and some from the other 

 might be victorious. But in the course of time, the 

 forms dominant in the highest degree, wherever pro- 

 duced, would tend everywhere to prevail. As they pre- 

 vailed, they would cause the extinction of other and 

 inferior forms ; and as these inferior forms would be 

 allied in groups by inheritance, whole groups would 

 tend slowly to disappear ; though here and there a 

 single member might long be enabled to survive. 



Thus, as it seems to me, the parallel, and, taken in a 

 large sense, simultaneous, succession of the same forms 

 of life throughout the world, accords well with the prin- 

 ciple of new species having been formed by dominant 

 species spreading widely and varying ; the new species 

 thus produced being themselves dominant owing to in- 

 heritance, and to having already had some advantage 

 over their parents or over other species ; these again 

 spreading, varying, and producing new species. The 

 forms which are beaten and which yield their places to 

 the new and victorious forms, will generally be allied in 

 groups, from inheriting some inferiority in common ; 

 and therefore as new and improved groups spread 

 throughout the world, old groups will disappear from 

 the world ; and the succession of forms in both ways 

 will everywhere tend to correspond. 



There is one other remark connected with this subject 

 worth making. I have given my reasons for believ- 

 ing that all our greater fossiliferous formations were 

 deposited during periods of subsidence ; and that 



