PROCESSES OF EVOLUTION 



251 



further broken up. Thus for maize, which is probably the most wide- 

 spread crop in the world, ranging from 58° N. to 40° S., with an enor- 



mous number of variants, some of which ripen in seventy days while 

 others require eleven months, and a range in size between 60 cm. and 

 6 cm., subsidiary centres of diversity can be distinguished for the sub- 



