228 THE FORMATION 



with indigenous and autochthonous, while others have regarded it as an 

 antonym of exotic. In its proper sense, endemic refers to distribution, and 

 not to origin. Its exact opposite will be found then in Fenzl's term poly- 

 demic, dwelling in several regions. Indigenous (autochthonous) and exotic, 

 on the contrary, denote origin, and are antonyms, indigenous signifying 

 native, and exotic foreign. As Drude has shown, endemic plants may be 

 either indigenous, as in the case of those species that have never moved out 

 of the original habitat, or exotic, as in the much rarer instances "where a 

 polydemic species has disappeared from its original home and from all 

 regions into which it has migrated except one. It is understood that not 

 all indigenous or exotic species are endemic. The proportion of endemic to 

 polydemic species is a variable and somewhat artificial one, depending upon 

 the size of the divisions employed. 



277. Causes. The primary causes of endemism are two, lack of migra- 

 tion and presence of barriers. Since distributive agents are practically 

 universal, lack of migration corresponds essentially to immobility, a fact 

 which decreases the difficulty of ascertaining the immediate causes of 

 endemism in any particular species. Either immobility or a barrier may 

 produce endemism: extremely immobile plants, for example, liliaceous 

 species propagating almost wholly by underground parts, are as a rule 

 endemic, while alpine plants and those of oceanic islands are endemic in 

 the highest degree, regardless of their mobility. When the two conditions 

 act concomitantly upon a species, endemism is almost inevitable. It can not 

 be supposed, however, that immobility or natural barriers alone, or the 

 concomitance of the two, must invariably give rise to endemic species; the 

 most immobile plant may be carried into another region by unusual or 

 accidental agencies, or the most formidable barrier to migration may be 

 overcome by the intensity of an agent or through the action of man. En- 

 demism is also brought about by the modification of species; new or 

 nascent species are as a rule endemic. Whether they will remain endemic 

 or not will depend upon the perfection of their contrivances for dissemina- 

 tion and upon the presence of barncrs to migration or ecesis. Finally, as 

 Drude was the first to point out, the disappearance of a polydemic species in 

 all regions but one, owing to the struggle for existence or to changed 

 physical conditions, will result in endemism. 



278. Sisrnificance. Endemism is readily recognized by methods of 

 distributional statistics, applied to areas limited by natural barriers to 

 migration or ecesis. For political areas, it has no significance whatever, 

 unless the boundaries of these coincide with barriers. It determines in the 



