Chapter III —37— 



On Areas 



of tuber-bearing irises but also three of the sections of rhizome-bear- 

 ing irises are hmited in their distribution to the Mediterranean Basin 

 Moreover, ahnost all the other sections have representatives here 

 These circumstances force one to presume that the center of origin of 

 the area of the genus Iris must be located in the Mediterranean Basin 

 This conclusion is confirmed by cytological data. Precisely in the 

 Mediterranean Basin are concentrated those species with the lowest 

 chromosome numbers (n = 8, 9, 10, 11), whereas American species with 

 the highest chromosome numbers have areas located at the greatest 

 distance from this center. Moreover, the tuber-bearing species of 

 irises, being the most ancient species and having an area confined to 

 the Mediterranean Basin, have the lowest chromosome numbers 

 Hence, if, in determining the location of the centers of areas, we utiHze 

 cytological data for those genera the species composing which have 

 different chromosome numbers, we acquire an additional method 

 facihtatmg, in combination with other methods, the solution of this 

 diflicult problem. 



If, after a species has died out over a considerable portion of its 

 area, favorable conditions should reoccur, the species may renew its 

 dispersal from those retreats where it preserved its habitats. For 

 example, many species hved through the Ice Age in restricted localities 

 which served as retreats for them and whence, in inter- and post-glacial 

 periods, they renewed their dispersal. In such cases these retreats are 

 known as centers of dispersal (centres de dispersion— Jeannel and 

 JoLEAUD, 1924) or centers {regions) of preservation (Erhaltungsgebiete— 

 Ikmscher, 1929). 



In determining the location of the center of an area great caution 

 must be observed, since if any factors whatsoever favoring or hindering 

 the distribution of species are not taken into account, entirely incorrect 

 conclusions may be drawn. For instance, Palmgren (1927), on the 

 basis of the character of the distribution of species on the Aland Islands 

 draws conclusions as to the extent of their penetration into the terri- 

 tory of these islands and the general direction of their migration. He 

 considers that, in case a species is distributed uniformly within the 

 limits of a given territory, we cannot obtain any facts as regards its 

 former migrations from its present distribution. By uniform distri- 

 bution he means the approximately equal frequency of occurrence of a 

 species in all parts of the given territory and, consequently, the ab- 

 sence of any perceptible concentration of stations in any one part. If 

 on the other hand, the frequency of occurrence of a species grows 

 clearly less or greater in some definite direction, this indicates, in his 

 opinion, the direction of migration of the species. Thus, by his 'investi- 

 gation of the Aland Islands Palmgren found that, in addition to 

 uniformly distributed species, there are three other categories of 

 species, which, in contradistinction to the former, shed some light on 

 their origin on the islands. The first category embraces species with a 

 clear decrease in the frequency of their occurrence toward the east 

 which gives grounds for considering that they migrated from the west' 

 This group of species is the largest in point of numbers in the flora of 

 the Aland Islands. To the second category belong a few species, the 

 frequency of occurrence of which decreases toward the west, and which 



