332 FORMS OF LIFE CHANGING 



greatly struck those admirable observers, MM. 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, 

 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 cur- 

 rents, climate, or other physical conditions, 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 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 num- 

 ber 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 terri- 

 tories of other species, should be those which would have the 

 best chance of spreading still farther, 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 geographical 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 succeed 

 in spreading and would ultimately prevail. The diffusion 

 would, it is probable, be slower with the terrestrial inhabit- 

 ants of distinct continents than with the marine inhabitants 

 Qf fcbe continuous sea^ 'We might therefore expect to fin^ 



