GEOLOGICAL SUCCESSION OF ORGANIC BEINGS 111 



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 having 

 some advantage over older forms; and the forms, which 

 are already dominant, or have some advantage over the 

 other forms in their own country, give birth to the great- 

 est number of new varieties or incipient species. We 

 have distinct evidence on this head, in the plants which 

 are dominant, that is, which are commonest and most 

 widely diffused, producing the greatest number of new 

 varieties. It is also natural that the dominant, varying, 

 and far-spreading species, which have already 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 other 

 new varieties and species. The process of diffusion would 

 often be very slow, depending on climatal and geograph- 

 ical changes, on strange accidents, and on the gradual 

 acclimatization of new species to the various climates 

 through which they might have to pass, but in the 

 course of time the dominant forms would generally suc- 

 ceed in spreading and would ultimately prevail. The 

 diffusion would, it is probable, be slower with the ter- 

 restrial inhabitants of distinct continents than with the 

 marine inhabitants of the continuous sea. We might 

 therefore expect to find, as we do find, a less strict de- 

 gree of parallelism in the succession of the productions 



of the land than with those of the sea. 



—Science— 22 



