FORMS OF LIFE CHANGING 375 



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 gen- 

 eral 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 Bar- 

 rande 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 arc formed by having some 

 advantage over older forms ; and the forms, which are al- 

 ready 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 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 climatal and geo- 

 graphical changes, on strange accidents, 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 inhabi- 

 tants of the distinct continents than with the marine inhabi- 

 tants of the continuous sea. We might therefore expect to 

 find, as we do find, a less strict degree of parallelism in the 



