TEMPEEATE FLOKAS 



27 



range of a species, often so that it exists only in two more or 

 less isolated areas, then to diminish the number of individuals 

 in these areas, and finally to reduce them to a single group 

 which ultimately succumbs to an increased stress of competi- 

 tion or of adverse climatal changes, when a species which 

 may have once been flourishing and widespread akogether 

 ceases to exist. The rarity of a species may thus be 

 considered as an indication of approaching extinction. 



Numerical Distribution of Plants hi Britain 



We will now give a few numerical statements as to the 

 comparative abundance of the species of plants in large and 

 small areas in various parts of the world, such facts having a 

 special application to the theory of evolution. The 55 

 counties of England and Wales (counting the three Ridings 

 of Yorkshire as counties) have usually areas from 500 to 

 2500 square miles; and a considerable number of them 

 have had their plants enumerated in special catalogues or 

 floras. The following are the approximate numbers of the 

 flowering plants in a few of these : — 



