l88 introduction to plant geography [chap. 



Discontinuous Ranges 



In discontinuous or disjunct ranges the plants are separated by 

 wider gaps than the dispersal capacity of their propagules would 

 normally bridge. Sometimes the distinction from the so-called 

 continuous ranges is doubtful, in being a mere matter of degree, but 

 often a taxon will inhabit two or more widely separated areas whose 

 elucidation may be a difficult matter. In many cases, however, 

 areas once thought to be entirely distinct have been found to be 

 otherwise on exploration of intervening tracts, in which the plant 

 or plants in question have appeared, and consequently supposed 

 gaps have been closed {cf. Fig. 62). In yet other cases, ranges may 

 if desired be considered continuous provided there is an absence 

 of any suitable habitat between the colonized areas, though here 

 again a discreet sense of proportion must be exercised and such 

 major barriers as oceans and ice-caps often recognized. 



The above discussion affords instances emphasizing the need for 

 caution in describing the distribution of a particular plant — especially 

 if it is of a small or insignificant nature, or is adapted to a limited 

 range of habitat conditions and consequently has a ' fragmented ' 

 and complex topography. But provided these warnings are borne 

 in mind and no inflated body of theorizing is based on unwarranted 

 supposition, the concept of discontinuity of area is a very real one 

 and has greatly stimulated research and philosophical speculation 

 in plant geographical and allied fields. 



As for the main causes of discontinuity (apart from the controversial 

 extremes of sudden long-distance dispersal or ' historical ' wiping 

 out in intermediate areas, both of which have obviously taken place 

 in the past), they are usually environmental in being due to particular 

 topographic, climatic, edaphic, or biotic characteristics which lead 

 to areas being separated from each other by tracts of different 

 character. This sets aside for the time being the possibility of 

 polytopic origin {see pp. 206 et seq.). 



We should mention some of the general types of discontinuity, 

 regardless of cause. An area is described as (i) dijfiise when it is 

 broken up into small, more or less numerous and equal parts ; 

 (2) bipartite when it is composed of only two separate parts in the 

 same hemisphere, one of which is extensive and forms the main 

 part and the other of which is subordinate ; (3) bipolar when it is 

 composed of two parts widely separated in the northern and southern 

 hemispheres ; (4) altitudinal when it is composed of one part 



