INTANGIBLE BARRIERS 455 



readily transported for long distances. Endless examples 

 of this same sort of restraint of species within limits 

 not established by material barriers might be cited. From 

 this one may conclude that they do not spread because 

 the conditions of soil and climate are not suitable to them 

 beyond the limits of their natural ranges. A careful 

 consideration of the factors shows that where no tangible 

 barriers exist, and the region has an equally distributed 

 rainfall, species spread over a far greater extent east and 

 west than north and south. This suggests that tempera- 

 ture may be one of the conditioning factors, a supposition 

 which can readily be verified experimentally in many 

 cases. There will thus be a tendency for the world to be 

 divided into temperature belts or zones each of which is 

 adapted to particular species. But since the rainfall is 

 unlikely to be uniform throughout the east and west 

 extent of such zones, they will again tend to be divided 

 into smaller areas on the general basis of the moisture 

 factor of climate. Of course it is well known to every- 

 one that on account of the position of mountains and 

 large bodies of water and the direction of the winds, 

 these zones of temperature are not straight horizontal 

 belts extending uniformly around the earth but are in 

 fact bent far to the north or south, widened out or greatly 

 narrowed according to the various positions of these 

 physical factors. In fact one may often go south and up 

 a mountain and find the same changes in temperature 

 and vegetation that he would find in going north over 

 a very much longer distance on the level. This is due, 

 of course, to the well known fact that temperature 

 decreases with altitude as well as with latitude. Nor 

 are the divisions dependent on moisture any more regu- 

 lar, since rainfall also usually increases with altitude, 

 especially on the windward slopes of mountains. In con- 

 sidering facts of this sort it should not be forgotten that 

 these differences of climate really act as conditions for 

 favorable development and only occasionally as genuine 



