292 ON THE ORIGIN OF SPECIES 



when we treat of the present distribution of organic 

 beings, and find how slight is the relation between the 

 physical conditions of various countries, and the nature 

 of their inhabitants. 



This great fact of the parallel succession of the forms 

 of life throughout the world, is explicable on the theory 

 of natural selection. New species are formed by new 

 varieties arising, which have some advantage over 

 older forms ; and those forms, which are already domi- 

 nant, or have some advantage over the other forms in 

 their own country, would naturally oftenest give rise to 

 new varieties or incipient species ; for these latter must 

 be victorious in a still higher degree in order to be pre- 

 served and to survive. We have distinct evidence on 

 this head, in the plants which are dominant, that is, 

 which are commonest in their own homes, and are most 

 widely diffused, having produced the greatest number 

 of new varieties. It is also natural that the domi- 

 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 species, 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 



