252 AN INTRODUCTION TO MODERN GENETICS 



sidiary groups, e.g. soft maize is most variable in Peru and Bolivia, 

 hard maize in South Mexico and Central America.^ 



Vavilov's theory is, of course, only generally true and may be ex- 

 pected to break down in some cases. For example, Turesson^ has 

 pointed out that the centres of diversity of some Eurasian plants is in 

 Finland and Sweden, which were probably beneath the polar ice cap 

 (during the Ice Age) when the species originated. He suggests that their 

 present great diversity of forms may be due to a climatic similarity with 

 the original centres of origin. 



1 Kuleshov 1933. ' Turesson 193 1- 



