Chap. X. THROUGHOUT THE WORLD. 325 



de Verneuil and d'Archiac. After referring to the 

 parallelism 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, can- 

 not be owing to mere changes in marine currents or 

 other causes more or less local and temporary, but de- 

 pend 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 

 conditions, 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 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- 



