826 
GEOLOGICAL SUCCESSION, 
Chap. X. 
nant^ varying, and far-spreading species, wliicli already 
have 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 new varieties and species. The process 
of diffusion may often be very slow, being dependent 
on climatal and geographical changes, or on strange 
accidents, but in the long run the dominant forms will 
generally succeed in spreading. The diffusion would, it 
is probable, be slower with the terrestrial inhabitants of 
distinct continents than with the marine inhabitants of 
the continuous sea. We might therefore expect to find, 
as we apparently do find, a less strict degree of parallel 
succession in the productions of the land than of the sea. 
Dominant species spreading from any region might 
encounter still more dominant species, and then their 
triumphant course, or even their existence, would cease. 
We know not at all precisely what are all the conditions 
most favourable for the multiplication of new and domi¬ 
nant species; but we can, I think, clearly see that a 
number of individuals, from giving a better chance of 
the appearance of favourable variations, and that severe 
competition with many already existing forms, would be 
highly favourable, as would be the power of spreading 
into new territories. A certain amount of isolation, 
recurring at long intervals of time, would probably be 
also favourable, as before explained. One quarter of 
the world may have been most favourable for the pro¬ 
duction of new and dominant species on the land, and 
another for those in the waters of the sea. If two great 
regions had been for a long period favourably circum¬ 
stanced in an equal degree, whenever their inhabitants 
met, the battle would be prolonged and severe ; and 
some from one birthplace and some from the other 
might be victorious. But in the course of time, the 
