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 w^orld, 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- 
