250 AN INTRODUCTION TO MODERN GENETICS 



suggests that the species did not originate by the gradual transformation 

 of the gene-complex of a population, but that the new species appeared 

 quickly in a few individuals. 



3. The Local Origin of Important Groups of Plants 



A species which has originated suddenly and in a localized area in the 

 way postulated by Willis will then spread and undergo further evolu- 

 tion and may give rise to a large and varied group of descendants. 

 Vavilov^ pointed out that the greatest number of variants should be 

 found in the region in which the group first originated, where it has 

 had longest to produce new types. The places of origin of such groups 

 can therefore be identified as the regions where the greatest number of 

 variants can be found. Vavilov and his associates have conducted very 

 extensive collecting expeditions to many parts of the world, chiefly in 

 connection with attempts to find new races of crop plants which may 

 be used to improve the yield of agriculture in the U.S.S.R. Although an 

 enormous number of specimens have been collected, including for 

 instance, fourteen new species of potato, only one species of which was 

 previously known in Europe, there are still very many gaps in our 

 knowledge; we have so far had to rely, in this field, almost solely on 

 the efforts of the one country which has serious economic reasons for 

 trying to improve its crop yields. 



On the basis of what is known at present, it seems Hkely that most of 

 the important crop plants originated in a few regions. The conditions 

 which caused each of these regions to be the birthplace of several large 

 groups of important crops are not imderstood. Vavilov distinguishes 

 seven such centres : 



(la) South- West Asia, including North- West India, for soft wheats, 



rye, several Leguminosae and many fruit trees, 

 (i^) India for rice, sugar cane, and many tropical crops. 



(2) East China for fruit trees and soya bean. 



(3) Abyssinia for hard wheats, barley, and leguminous crops. 



(4) Mediterranean regions for the olive and certain forage plants. 



(5) South Mexico and Central America for maize and upland 



cotton. 



(6) Peru and Bolivia for the potato. 



More detailed study shows that some of these primary centres can be 

 ^ Vavilov 1928, 1932. 



