234 EVOLUTION OF PLANTS 
rather than a continental climate, and the same is true 
of the northern parts of the Pacific coast of North 
America. : 
Within the tropics there are few genera common to 
the Old and New Worlds, although many families, e.g. 
palms, orchids, aroids, and others, are abundantly repre- 
sented in both regions, but usually by distinct genera. 
Where a genus is common to both regions, it is usually 
one which has a wide range through the north tem- 
perate regions as well; e.g. the orchidaceous genus 
Cypripedium and many genera of ferns, e.g. Polypodium, 
Adiantum. 
The flora of isolated regions, seen in its most extreme 
form in such oceanic islands as the Hawaiian Islands 
and St. Helena, is always exceedingly peculiar, owing to 
the long intervals at which new forms are introduced and 
the modifications which most of these subsequently have 
undergone on account of their changed environment. 
Such regions always contain a large proportion of en- 
demic or peculiar species. While wide’ expanses of 
ocean offer the most effective barriers to the distri- 
bution of most plants, high mountains and deserts also 
act as efficient checks to the migration of plants, and 
a very different flora may exist upon opposite slopes 
of high mountain ranges separated by only a few miles 
of actual distance. A marked instance of this is seen 
in the character of the plants upon the eastern and 
western slopes of the Andes. In the United States 
the almost totally different character of the plants 
of the Atlantic and Pacific coasts, except in the north- 
ern regions where occur a number of the sub-polar 
types common to the whole northern zone, illustrates 
