CAIN : CENTER OF ORIGIN 137 



the central and eastern Andes on which pre-Cenozoric plant developments 

 might well have occurred, and from which much of the modern Andean flora 

 must have been derived. 



Another exception to the center of origin being where the greatest dififeren- 

 tiation of a type exists is that resulting from polyploid complexes. Polyploidy 

 tends to break down genetic barriers between species with a resultant produc- 

 tion of a large number of varieties and species (Stebbins, 1940). Examples of 

 such complexes include Crepis, Zauschneria, Rosa, Rubus, and sections of 

 Potentilla, Antennaria, and Taraxacum, and dysploidy may increase the intri- 

 cacy of the complex. Goodspeed and Bradley (1942) note the conclusion of 

 Kostofif (1938) that amphidiploids from F^ hybrids may give rise to mono- 

 morphic species, but in other cases, if a series of segregated forms can survive, 

 a polymorphic species is produced. Inconstant amphidiploidy may originate a 

 series of adaptable forms and provide suitable material for natural selection. 

 In every case, according to Stebbins, the majority of the basic diploids are rel- 

 atively restricted in area, while most of the widespread types are polyploid. He 

 says, "The center of distribution of the diploid species of a polyploid complex 

 is naturally the center of variation of the complex as a whole . . . the diploids 

 tend to occupy the older, more stable habitats. This makes the study of poly- 

 ploid complexes very important from the standpoint of plant geography." Such 

 centers of variation as are due to hybridization and polyploidy may develop at 

 the center of origin of a genus, but that is not necessarily the case. The Amer- 

 ican species of Crepis have such a center in the Pacific Northwest, but the stock 

 immigrated from the Asiatic center of the genus (Babcock and Stebbins, 1938) . 



A third type of exception to the criterion consists of such phylogenetic 

 stocks as have developed a center of variation at the center of origin, in the 

 orthodox manner, but which have suffered a decimation of the group at the 

 center as a result of physiographic and climatic changes. Through emigration 

 and extinction due to climatic and physiographic changes the variety of types 

 may be reduced in one region so that a secondary center comes to contain more 

 variety. 



Hulten (1937) has also come to the conclusion that "it must ... be unsafe 

 to assume that a plant originates in the place where it has its most numerous 

 relatives. In most cases such a consideration will perhaps be correct, but in 

 others it must be misleading." He illustrates this point by reference to old, 

 widespread, arctic-montane species. "It is natural therefore that in different 

 parts of the area of a Linnaean species considerably differentiated races should 

 be found. The area has repeatedly been split up, during the glacials under the 

 influence of a cold climate in the north and a pluvial one in the south, and dur- 

 ing the interglacials under the influence of drought and heat. Each of these 

 agencies must have caused a selection of biotypes in its particular direction . . . 



