78 PLANT GEOGRAPHY 



by comparatively slight changes of circumstances, will 

 offer an effective barrier against the spread of a xero- 

 phytic species; and in the same way the dense shade of 

 a forest apart altogether from its action as preoccupying 

 the ground will check invasion by sun-loving (ombro- 

 phobe) species. Such a forest-barrier would, however, 

 be traversed if interrupted by open spaces. 



BIOLOGICAL BARRIERS. Among what are termed 

 biological barriers may be reckoned the preoccupation 

 of the ground, especially in the case of " close " or dense 

 formations, such as the strong-growing social grasses of 

 a meadow or the jungle or " rain-forest " of equatorial 

 regions; the absence of suitable "host" species as 

 limiting the occurrence of parasites; the absence of 

 necessary pollinating insects, already alluded to; and 

 the presence of serious fungal diseases. These will 

 generally be but partial and temporary obstacles to 

 migration. 



As an example of the last-mentioned biological barrier 

 mention may be made of the Larch-canker (Dasyscypha 

 calycina Fckl.). This fungus occurs sporadically in the 

 mountain home of the Larch; but in our moist plains 

 the tree is less resistant, and the disease becomes 

 epidemic. 



CHAPTER VII 



INSULAR FLORAS 



No class of facts is of greater importance as illustrating 

 the principles of botanical geography than that relating 

 to the floras of islands. 



Islands fall into two very distinct classes, differing 

 widely in origin, geological structure, and the character 

 of then- native plants and animals. They are either 

 continental or oceanic. The former are seldom at any 

 great distance from the continent from which they have 

 been severed. They are of varied geological structure, 

 partly of stratified and partly of igneous rocks, and their 

 plants are in the main identical with those of the 

 continent. Continental islands are, however, subdivided 

 into two subclasses, recent and ancient. Recent con- 

 tinental islands, such as the British Isles, Borneo, Java, 



