COMPETITIOxN AND ECESIS 269 



endemic when it is found in a single formation, and polydemic 

 when it occurs in two or more. 



Endemism is due to the lack of migration, or the presence of 

 barriers. Plants which are extremely immobile, e.g., those that 

 migrate slowly by underground offshoots, are often endemic, 

 ^lany species of alpine regions and of oceanic islands are endemics, 

 owing to the check offered by barriers to their spread. Immobile 

 species which are surrounded by barriers are almost inevitably 

 endemic. However, the most immobile plant may be scattered 

 widely by new or unusual agencies, or the most formidable barrier 

 may be sometimes overcome, especially through the activities of 

 man. Endemism is also a direct result of evolution, since new 

 forms as a rule are endemic at first. Whether they remain en- 

 demic depends upon their mobility and upon the presence of 

 Ijarriers to migration or ecesis. An endemic may also arise in 

 rare instances l^y the disappearance of a polydemic in all regions 

 or formations but one, owing to competition or to changed physi- 

 cal conditions. 



A species usually becomes polydemic by virtue of being carried 

 into two or more different formations or regions and becoming 

 established there. A plant may also be polydemic in conse- 

 quence of having originated in two or more places. 



Experiment 68. Barriers and endemism. IMake a list of the species 

 found ill two or more neighboring ravines, separating those found in 

 one ravine from those that occur in all. Make a similar list for the 

 intervening hills or ridges. Point out the action of barriers in several 

 of the most striking instances. 



