THE FACTORS OF DISTRIBUTION 77 



the most remote from continents, consist largely of 

 species indigenous to the nearest continent, the propor- 

 tion of the species of the continent which succeeds in 

 reaching the island is small, and consists mainly of 

 forms with small, light seeds, or with fleshy fruits 

 attractive to birds, or of strand-plants which can with- 

 stand the action of sea-water. Sea may, in fact, be 

 said to form a barrier in proportion to its width. No 

 species or even genus of Palms is, for example, common 

 to the tropics of the Old and New Worlds. On the other 

 hand, sea assists the distribution of marine Algae, while 

 the wider a mass of land intervening between two ocean 

 areas, the less in common will there generally be between 

 the Algae of those seas. Even long and wide rivers, 

 such as the Obi, serve to divide floras. 



MOUNTAIN-CHAINS. Mountains that are lofty enough 

 to transcend the vertical limits of any of those altitu- 

 dinal zones to which reference has already been made 

 will obstruct the migration of species belonging to lower 

 zones, whilst snow-capped ranges will, of course, con- 

 stitute yet more complete barriers. Macro therms will 

 have great difliculty in passing such a barrier. The 

 direction of such mountain-barriers may be most sig- 

 nificant. In Eurasia they run east and west, one result 

 of which has been that the Tertiary flora, driven south 

 by the increasing cold of the Glacial Epoch, had mainly 

 to escape eastwards rather than southwards, and was 

 largely exterminated, only one species of Palm, one 

 Laurel, one Myrtle, and one Fig surviving in the Mediter- 

 ranean flora of to-day. In America, on the other hand, 

 the mountain-barrier running north and south, the 

 warmer Tertiary flora represented by such genera as 

 Magnolia, Liriodendron, Liquidambar, Sequoia, and 

 Tax odium, was free to migrate southward, and to return 

 as the climate once more improved. 



DESERT. That a broad stretch of desert, with scarcely 

 any water-supply save at rare oases, should act as an 

 effective barrier to all plants except xerophytes is 

 obvious. The Sahara forms a very complete barrier 

 between the Soudanese and Mediterranean floras; but 

 here a considerable difference of latitude is involved, 

 and, for some distance also, the lofty range of the Atlas. 



VEGETATION. A wide area of marsh, covered with its 

 characteristic vegetation, although liable to be drained 



