THE FACTORS OF DISTRIBUTION 75 



On the other hand, many species have an extremely 

 restricted distribution, or small area of distribution. 

 This is frequently the case with species in insular and 

 mountain floras. This fact suggests that such species 

 may have been evolved where they are now found. 

 Their not having spread further may be attributed to 

 immobility; and, when transplanted, they often do not 

 refuse to grow elsewhere. The earth, in fact, by no 

 means produces everywhere the species best adapted to 

 each particular region, for introduced plants, such as 

 Water-cress and Tri folium rep ens L. in New Zealand, 

 will often oust native species. 



PHYSICAL BARRIERS. The distribution of most species 

 and their power of invading new areas is, in fact, 

 limited by barriers. These may be complete or partial, 

 permanent or temporary, physical or biological. Among 

 physical barriers we may class differences of climate and 

 of soil, stretches of sea and, for aquatic plants, of land, 

 wide rivers, deserts, mountain-chains, and masses of 

 vegetation in so far as their action is merely physical, 

 as in retaining a large amount of stagnant water, or 

 in producing a dense shade. Most of these physical 

 barriers are of a permanent character. 



CLIMATE. There is unquestionably no more potent 

 factor in the separation of floras than differences of 

 climate, by which, as we have seen, is mainly meant 

 differences in the amount and seasonal distribution of 

 atmospheric temperatures and moisture. As climates 

 are limited horizontally and vertically, and species have 

 but little power of acclimatisation, or adapting them- 

 selves to climates other than those to which they are 

 habituated, each species may be said to have a poleward 

 and an equator- ward or north and south limit each 

 being practically an isotherm and a lower and an 

 upper vertical limit. Migration will thus be possible to 

 greater distances (unless other serious barriers intervene) 

 in an east and west direction than meridionally. As 

 there is no reason to suppose any great displacement 

 of the Poles and the Equator at least within those 

 geological times with which we are concerned it is easy 

 to see that the vegetation of the earth must have always 

 been separable into three great latitudinal zones or 

 ancient floras northern, tropical, and southern. 



The Northern is now characterised by its needle- 



