CH. V] HINDER THE DISPERSAL OF SPECIES 41 



in the near neighbourhood of the mountains. In north-central 

 Europe, on the other hand, the cHmate slowly becomes drier 

 Avith fair regularity in passing from England to the Urals and 

 then becomes suddenly much drier. In the Scandinavian 

 peninsula, the mountains he more across the wind, and Sweden 

 is much drier than Norway. 



Farther south, there is a great belt of more or less dry and 

 desert country, almost round the world in the northern, much 

 less marked in the same latitudes of the southern, hemisphere 

 and between these two drier regions, which oppose all but im- 

 passable barriers, lies the wet zone of the equatorial tropics 

 where the climate is usually damp, and often very rainy through 

 a great part of the year, though there are alternations of drier 

 and wetter periods. 



If a country be flat, or nearly so, as, for instance. North Europe 

 from England to the Urals, the rai7ifall gradually falls off as 

 one goes inland from the sea, but only in averages over a number 

 of years. If, for example, at a series of stations, Avorking inland 

 from the sea, the rainfall average 50, 45, 40, 35 inches, it^'is quite 

 possible, if not even probable, that in some years the fall at the 

 station farthest inland may be 50, or in others that the fall at 

 the station nearest the sea may be only 35. Unless, therefore, 

 plants are suited to a great range in the amount of rainfall, they 

 cannot hope to succeed in most stations, and it also becomes 

 doubtful when and where the rainfall reaches an absolute 

 maximum or minimum which causes it to be an ecological 

 barrier. It is also highlj^ unlikely that this point will be the 

 same for any two species. That there is such a barrier seems not 

 improbable when we consider the difference in flora between 

 the steppes of Russia and the British Islands, but where it 

 exists for any single species we are unable to state. 



If, however, as very often happens, a mountain chain stand 

 athwart the prevailing or most frequent winds, there may be a 

 sudden change in the rainfall. The damp air from the sea, striking 

 the mountains, is forced upwards and cooled, parting with much 

 of its moisture; then as it descends upon the other side, it be- 

 comes warmed, and thereby much drier. In Ceylon, for example, 

 the south-west monsoon blo^vs for about six months over a vast 

 expanse of ocean, and reaches the island a saturated wind. 

 Meeting the mountains, it deposits an enormous rainfall (over 

 100 inches at the foot of Adam's Peak), and upon the eastern 

 side (they reach 8000 feet) becomes a dry scorching wind, deposit- 



