[Chap. XXXVI ORIGIN OF PLANTS USED BY MAN 425 



more abundantly than others. Continued variation accompanied by selec- 

 tion through many thousands of years resulted in cultivated varieties 

 that differ in many ways from their wild ancestors. 



There is good evidence that long before historic times man began 

 to notice and select desirable variants of the plants on which his ex- 

 istence depended. The primitive tribes in the interior of New Guinea, 

 whose civilization todav is considered by ethnologists to correspond to 

 the late Stone Age, distinguish and name the numerous varieties of sugar 

 cane they are cultivating. 



Man may also unconsciouslv select and promote the propagation of 

 certain varieties of cultivated plants. Owing to the occurrence of natural 

 variations, a field of crop plants usuallv consists of a mixture of se\ eral 

 varieties just as the population of a citv is composed of manv different 

 kinds of people all of whom are members of the same species. Experi- 

 ments have shown that in a mixed population of crop plants some vari- 

 eties may produce more seeds than others, have a greater number of 

 ripe seeds at harvest time, or have seeds that germinate more readih- 

 when planted. Consequently in a mixed population of plants that propa- 

 gate by seeds, certain varieties mav gradually increase and others de- 

 crease in abundance without anv conscious selection on the part of 

 man (Table 15). 



Table 15. Changes in a Mixed Crop of Wheat During a Five-year Period 

 Without Anv Conscious Selection bv the Farmer. Percentages of the vari- 

 eties and species in the crop during the first season of the test are given in 

 column A; at the end of five years in column B. 



Wheat Varieties A B 



The evolution of cultivated plants has occurred over such a long period 

 of time and resulted in such marked changes in the plants that botanists 

 have had great difficulty in discovering the wild ancestors of some of 



