2o6 INTRODUCTION TO PLANT GEOGRAPHY [CHAP. 



by their relic character and geographical isolation, their small 

 amplitude of variation and narrow restriction to particular ecological 

 conditions, their relatively small chromosome number, and their 

 generally retrogressive nature. On the other hand, neo-endemics, 

 being secondarily derived types, commonly have larger chromosome 

 numbers ; they also tend often to be relatively aggressive. This is 

 especially the case when they are rich in biotypes, for example owing 

 to hybridization. 



Apart from the so-called ' pseudo-endemics ' that have been 

 encountered only in one place and appear to be mutants, etc., and 

 unlikely to persist, there are the ecological endemics which have arisen 

 in relation to particular habitat conditions. 



Some endemics are confined to very limited areas, such as a single 

 small island or mountain peak, and may be called local endemics. 

 Such restriction is usually due (i) to their recent origin (so that 

 dispersal has only just begun), or (2) to their antiquity (so that 

 the area is a contracted one or even a ' last remnant '), or (3) to 

 their high specificity with regard to habitat conditions (which 

 prevail only at a given spot within the area that can be reached by 

 viable disseminules), or (4) to the impossibility of expansion (owing 

 to physico-geographical obstacles). 



It has been mentioned that isolated islands are often particularly 

 rich in endemics ; this is especially the case with those which are 

 at least some hundreds of miles from the nearest major land-mass, 

 and which may accordingly be termed oceanic. Although the dis- 

 tinction is far from definite, it is sometimes useful to think of other 

 islands, whose flora bears a closer relation to an adjacent land-mass, 

 and which are usually within at most a few hundred miles' distance 

 from it, as continental. These islands are phytogeographically like 

 fragments of continents or larger islands and are usually inhabited 

 by larger numbers of species than are comparable oceanic islands, 

 containing as they do both plants and animals for which transoceanic 

 transport seems virtually impossible. Some remote islands, such 

 as those surrounding Antarctica, are apt to be considered relics of 

 a formerly more extensive continent and hence scarcely oceanic. 



Polytopy and the Incidence of Areas 



Polytopy is the occurrence of a species or other taxon in two 

 or more separate areas, such species being termed polytopic or 

 polyendemic. The discontinuous ranges involving disjunct areas 



