Chap. XL] THBOUGHOUT THE WORLD. 103 



ly, in the above large sense, at distant parts of the world, 

 has greatly struck these admirable observers, MM. de 

 Verneuil and d'Archiac. After referring to the parallel- 

 ism of the palajozoic forms of life in various parts of 

 Europe, they add, " If, struck by this strange sequence, 

 we turn our attention to North America, and there 

 discover a series of analogous phenomena, it will appear 

 certain that all these modifications of species, their 

 extinction, and the introduction of new ones, cannot 

 be owing to mere changes in marine currents or other 

 causes more or less local and temporary, but depend on 

 general laws which govern the whole animal kingdom." 

 M. Barrande has made forcible remarks to precisely 

 the same effect. It is, indeed, quite futile to look to 

 changes of currents, climate, or otlier physical con- 

 ditions, as the cause of these great mutations in the 

 forms of life throughout the world, under the most dif- 

 ferent climates. We must, as Barrande has remarked, 

 look to some 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 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 

 greatest number of new varieties or incipient species. 

 We have distinct evidence on this head, in the plants 

 which are domihant, that is, which are commonest and 

 most widely diffused, producing the greatest number of 



