CiiAP. X. THROUGHOUT THE WORLD 305 



appear certain that all these modifications of species, tlieir ex- 

 tinction, and the introduction of new ones, cannot be owin^r to 

 move chani^es in marine currents or other causes more or less 

 local and temporarv, but de])cnd on general laws which govern 

 the whole animal kingdom." M. Barrande has made forcible 

 remarks to j)recisely tin; same eflect. 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 

 lift; throughout the world, under the most different climates. 

 Wc must, as Barrande has remarked, look to some special law. 

 AVe shall see this more clearly when Ave treat of the present 

 distribution of organic l)cings, and find how slight is the rela- 

 tion between the ])hysical conditions of various countries, and 

 the nature of their inhabitants. 



This great fact of the parallel succession of the forms of life 

 througlu^ut the world, is explicable on the theory of natural 

 selection. New species arc formed by having some advantage 

 over older forms ; and the forms, whicli are already dominant, 

 or have some advantage over the other forms in their own 

 country, would be the most likely to give birth to the greatest 

 number of new varieties or incipient sjiccies. 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 numljcr of new varieties. It is also natural that 

 the dominant, varying, and far-spreading species, which already 

 'lave 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 geographical changes, 

 on strange accidents, and on the gradual acclimatization of new 

 species to the various climates through which they might have 

 to jxiss, but in the course of time the dominant forms would 

 generally succeed in spreading, and would ultimately prevail. 

 Th(' diffusion would, it is probable, be slower with the terres- 

 trial inhabitants of distinct continents than with the marine 

 inhabitants of llie continuous sea. We might therefore expect 

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

 succession of the productions of the land than with those of the 

 sea. 



Thus, as it seems to me, the parallel, and,- taken in a large 

 sense, simultaneous, succession of the same forms of life through- 

 out the world, accords well witli tlic j>rinciple of new species 



