326 GEOLOGICAL SUCCESSION, Chap. X. 



nant, varying, and far-spreading species, which already 

 have invaded to a certain extent the territories of other 

 species, should be those which would have the best 

 chance of spreading still further, and of giving rise in 

 new countries to new varieties and species. The process 

 of diffusion may often be very slow, being dependent 

 on climatal and geographical changes, or on strange 

 accidents, but in the long run the dominant forms will 

 generally succeed in spreading. The diffusion would, it 

 is probable, be slower with the terrestrial inhabitants of 

 distinct continents than with the marine inhabitants of 

 the continuous sea. We might therefore expect to find, 

 as we apparently do find, a less strict degree of parallel 

 succession in the productions of the land than of the sea. 

 Dominant species spreading from any region might 

 encounter still more dominant sj)ecies, and then their 

 triumphant course, or even their existence, would cease. 

 We know not at all precisely what are all the conditions 

 most favourable for the multiplication of new and domi- 

 nant species ; but we can, I think, clearly see that a 

 number of individuals, from giving a better chance of 

 the appearance of favourable variations, and that severe 

 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 



