Chapter III —33— On Areas 



uplifting of the Alps and the Himalayas, which radically altered the 

 climatic conditions of the regions lying north of these mountain ranges), 

 shifting of sea basins (formation of deserts where formerly there were 

 seas and the formation of seas where formerly there was dry land), 

 separation from the continents of archipelagos and islands, which 

 formerly constituted a united whole, etc. 



Consequently, the present-day concentration of species only in rare 

 instances can reflect the actual center of origin of the genus; usually it 

 indicates the center not of the past but of the present development of 

 the genus. In view of this, conclusions made on the basis of data on 

 the migration of a genus from such a center of development are founded 

 on incorrect premises and in many cases are utilized for broad generali- 

 zations, which cannot be accepted without reservations, except upon 

 further verification. 



_ This explains why newer methods of determining the center of 

 origin of a genus are being sought, but it may be taken for granted 

 that these methods will be able to give full assurance as to the relia- 

 bility of the results obtained only in case they are confirmed by paleo- 

 botanic data. In the absence of the latter, conclusions drawn solely 

 on the basis of the present distribution of species will evoke doubts as 

 to their validity. Szymkiewicz (1934, 1936, 1937) has in recent years 

 made intensive studies with the aim of finding new methods of locating 

 the centers of areas and of tracing the development of floras. The 

 methods proposed by him may be summarized as follows: 



If we take as the center of origin of a genus that region where the 

 greatest number of its species are concentrated, we do not take into 

 account differences in the character of the areas of the various species, 

 as a result of which we compare figures that are phytogeographically 

 of unequal value. Szymkiewicz divides species, as regards the charac- 

 ter of their areas, into three categories: (i) endemic and subendemic, 

 the latter meaning species whose areas extend only slightly beyond the 

 boundaries of their primary natural regions; (2) species whose areas 

 embrace, in addition, a second natural region phytogeographically 

 identical to the first; (3) widely distributed species, in whose areas 

 the primary natural region occupies only an inconsiderable part. 

 These three categories of species, in judging as to the center and origin 

 of the area of a genus, provide data of unequal value, the first being of 

 greater significance than the second and the second greater than the 

 third. Szymkiewicz (1937) proposes, therefore, that the center of the 

 area of a genus should be established not on the basis merely of data 

 as to the total number of species but of data as to the number of species 

 in each of the three above-mentioned categories, and he points out 

 that by the latter method it is easier to detect a second center of con- 

 centration of species, in case there are two such centers. By way of 

 illustration are given below the data obtained by Szymkiewicz for the 

 genus Carex: 



