Chap. XI.] THROUGHOUT THE WORLD. 103 



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

 has greatly struck those admirable observers, MM. de 

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

 ism of the palaeozoic 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 other physical con- 

 ditions, as the cause of these great mutations in the 

 forms of life throughout the world, under the most 

 different climates. We must, as Barrande has re- 

 marked, 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 dominant, that is, which are commonest and 

 most widely diffused, producing the greatest number of 

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