COMPETITION AND ECESIS 



267 



habitats are much less effective in checking the spread of meso- 

 phytes during wet years or seasons. Many xerophytic habitats, 

 dunes, blowouts, gravel slides, prairies, etc., are barriers during 

 summer and autumn, but not during spring, when rains are fre- 

 quent and the surface remains moist. In the absolute sense no 

 barrier is complete, since, at one time or another, practically any 

 portion of the earth's surface is capable of supporting some kind 

 of vegetation. In connection with the natural spread of terres- 

 trial plants, however, it is convenient to distinguish partial barriers 

 from complete ones. In this sense large bodies of water and 



Fig. 107. An endemic plant, Polemonium spcciosum, found only in the 

 rock clefts of the highest Colorado peaks, such as Pike's, Gray's, etc. 



mountain ranges furnish the best examjiles of complete barriers. 

 Such barriers bring about the isolation of species, and tend to 

 restrict species to a single formation or region. 



285. Distance. Though hardly a barrier in the strict sense, 

 distance plays an important part in the amount of iiuasion. Tlio 

 effect of distance is clearly seen in migration, aspecially in the 

 case of a denuded area. The formations which toiidi the moa 

 usually furnish more than three-fourths of the species which soi-\o 

 to reclothe it. The chief reason for this is tlint the seeds of the 

 adjacent species are not so widely scattcretl !)> the time the ile- 



