j] TYPES AND AREAS OF NATURAL DISTRIBUTIONS 211 



done is to divide the species into apparently arctic and apparently 

 alpine groups ; and much the same applies to the subarctic or, as 

 it is sometimes called, subarctic-mountain element. Other geo- 

 graphical elements are fortunately apt to be less vague, examples 

 being the ^Mediterranean and the Atlantic ones. The broader types 

 of ecological groupings may also be included here when they char- 

 acterize geographical regions — as, for example, certain life-zones and 

 formations (or, better, biomes, which are climax formations of plants 

 and animals considered together, such as the characteristic Spruce- 

 Moose biome of most of the continental regions of Canada). Often 

 it is possible to decide on the geographical element to which a 

 species most likely belongs by locating its ' mass centre ' (of maximum 

 variation), for that is the part of its area which is most likelv to be 

 basic. 



(2) Genetic elements — grouped according to their region of origin 

 and accordingly reflecting the genesis of the flora. For this, detailed 

 monographic study of the groups involved is necessary, and so at 

 best it is usually possible to classify in this manner only a few chosen 

 species. These first two types of general element are considered 

 by some students to be the most important bases for floristic 

 analysis. 



(3) Migration elements — grouped according to the routes by which 

 they migrated to the region concerned. Examples of migration 

 routes are particular mountain passes, river valleys, and suitable 

 coasts. Unfortunately, species are apt to reach the domain of a 

 given flora by more than one route, so that the establishment of 

 particular migration elements is difficult or futile — though often well 

 worth attempting, as such elements may provide valuable clues to 

 the history of a flora. 



(4) Historical elements — grouped according to the time (such as 

 the postglacial climatic optimum period) when they became a part 

 of the flora concerned. Further examples are the so-called arctic- 

 Tertiary element of evergreen and deciduous trees, and the boreal- 

 Tertiary one which included such southerly members as Palms ; 

 these examples represent the inhabitants of the arctic and boreal, 

 respectively, regions in Tertiary times, while to their south, stretching 

 from southern England to Japan in those far-off days, lay the tropical 

 region of megatherms, i.e. plants adapted to high temperatures. 



(5) Ecological elements — grouped according to their immediate 

 habitat preferences. Most significant are the oceanic and continental 

 elements, embracing, respectively, those species which are adapted 



