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 



