106 
CIRCUMSTANCES FAYOURABLE 
Chap. IV. 
there supported, but the conditions of life are infinitely 
complex from the large number of already existing 
species; and if some of these many species become 
modified and improved, others will have to be improved 
in a corresponding degree or they will be exterminated. 
Each new form, also, as soon as it has been much im¬ 
proved, will be able to spread over the open and con¬ 
tinuous area, and will thus come into competition with 
many others. Hence more new places will be formed, 
and the competition to fill them will be more severe, on 
a large than on a small and isolated area. Moreover, 
great areas, though now continuous, owing to oscillations 
of level, will often have recently existed in a broken con¬ 
dition, so that the good effects of isolation will generally, 
to a certain extent, have concurred. Finally, I conclude 
that, although small isolated areas probably have been 
in some respects highly favourable for the production 
of new species, yet that the course of modification will 
generally have been more rapid on large areas; and 
what is more important, that the new forms produced 
on large areas, which already have been victorious over 
many competitors, will be those that will spread most 
widely, will give rise to most new varieties and species, 
and will thus play an important part in the changing 
history of the organic world. 
We can, perhaps, on these views, understand some 
facts which will be again alluded to in our chapter on 
geographical distribution; for instance, that the pro¬ 
ductions of the smaller continent of Australia have 
formerly yielded, and apparently are now yielding, 
before those of the larger Europseo-Asiatic area. Thus, 
also, it is that continental productions have everywhere 
become so largely naturalised on islands. On a small 
island, the race for life will have been less severe, and 
there will have been less modification and less exter- 
