GEOGRAPHICAL DISTRIBUTION 177 



interposition of barriers (mountains, broad deserts, salt 

 water areas, sudden changes of climate from one district to 

 the next, geological changes, natural selection, local adapt- 

 ation (the possession of a character useful in one country but 

 not in another), the dying out of occasional old species, the 

 arrival of a migrating species at its climate limit, et cetera. 

 But on the whole the endemic species, says Willis, are the 

 youngest. As an illustration of the operation of the hy- 

 pothesis of age and area, Small (I.e., p. 25-30) mentions 

 numerous Compositae which have limited distribution, 

 although there would seem to be practically no limit to the 

 distance their pappose seeds can be transported by wind. 

 They are limited (endemic) because they are young. 



According to another view, 1 endemic species are the 

 oldest species of a region; they are either relicts, and thus 

 very ancient, or they represent types which have been in 

 the region so long that their original characters have been 

 lost. The latter are indigenes, and are spoken of as 

 indigenous to the country. Endemics, according to 

 Sinnott, contain a greater percentage of trees than do 

 wides (or polydemics)- but, according to the same author, 

 trees and shrubs are older than herbs, and therefore the 

 endemic woody species must be older than the herbaceous 

 element of a given flora. The hypothesis of Willis demands 

 that herbs be considered as an older form of vegetation 

 than trees and shrubs, which, others argue, is contrary to 

 a mass of evidence. Trees are more common as endemics 

 (in Ceylon, e.g., twice as common), notwithstanding the 

 fact that they spread less rapidly than herbs. After its 



Sinnott, Edmund W. The "age and area" hypothesis and the 

 problem of endemism. Ann. Bot. 31:209-216. April, 1917. 

 2 "Wides" and "polydemics" are used as antonyms of endemics. 



12 



