Chap. XI. throughout the World. 299 



forms which are found only in the older underlying deposits, would 

 be correctly ranked as simultaneous in a geological sense. 



The fact of the forms of life changing simultaneously, in the 

 above large sense, at distant parts of the world, has greatly struck 

 those admirable observers, MM. de Yerneuil 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 modi- 

 " fications 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 

 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 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, 

 depending on elimatal and geographical changes, on strange acci- 

 dents, 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 distinct con- 

 tinents than with the marine inhabitants of the continuous sea. 

 We might therefore expect to find, as we do find, a less strict degree 



