CHAP. XI.] THROUGHOUT THE WORLD. 281 



special law. We shall see this more clearly 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 selec- 

 tion. 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 greatest 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, de- 

 pending on climatal and geographical changes, on strange accidents, 

 and on the gradual acclimatisation 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 succeed in spreading 

 and would ultimately prevail. The diffusion would, it is probable, 

 be slower with the terrestrial inhabitants of the distinct continents 

 than with the marine inhabitants of the continuous sea. We might 

 therefore expect to find, as we do find, a less strict degree of paral- 

 lelism in the succession of the productions of the land than with 

 those of the sea. 



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 principle of new species having been 

 formed by dominant species spreading widely and varying; the 

 new species thus produced being themselves dominant, owing to 

 their having had some advantage over their already dominant 

 parents, as well as over other species, and again spreading, vary- 

 ing, and producing new forms. The old forms which are beaten 

 and which yield their places to the new and victorious forms, wiU 

 generally be allied in groups, from inheriting some inferiority in 

 common; and therefore, as new and improved groups spread 

 throughout the world, old groups disappear from the world ; and 

 the succession of forms everywhere tends to correspond both in 

 their first appearance and final disappearance. 



There is one other remark connected with this subject worth 

 making. I have given my reasons for believing that most of our 

 great formations, rich in fossils, were deposited during periods of 

 subsidence; and that blank intervals of vast duration, as far as 



