THE ORIGIN OF CULTIVATED PLANTS 151 



in isolated fields; each batch of seed naturally produced a more 

 uniform crop than the heterogeneous one of the original field. 

 This sorting out of plants was continued again in each of the 

 new crops, plants of inferior quality being discarded arid seed 

 being chosen from the few superior plants. In time this process 

 of selection resulted in varieties with definite heritable traits; 

 one of these became the staple crop raised for a long time in 

 England and France. At this same period many other plant 

 growers in England, France and Germany were carrying out 

 systematic selection on other crop plants. Later, in this country, 

 such experimenters as Burbank carried on selection on a mam- 

 moth scale to produce superior plants. 



The two chief methods of selection used by plant breeders 

 today are mass culture and pedigree culture. In mass culture, 

 seeds are collected from a number of individuals which show 

 variations in the desired direction (such as larger wheat grains) 

 and are sowed together in a given area. From the plants grown 

 from these seeds, only the seeds of the most desirable variants are 

 again chosen. This process is repeated for several generations 

 until the individual plants reach the standards desired by the 

 grower. Mass culture has been particularly successful with cotton 

 and corn. There are objections to this method, however. It is 

 often slow, is difficult to separate plant characteristics due to 

 environmental factors from those due to hereditary factors, and 

 it does not isolate the best individuals but merely raises the 

 average for the group. Greatest objection is that this selection 

 process must be continued indefinitely or the race deteriorates. 



Pedigree culture meets some of these objections. This type of 

 selection has long been known to animal breeders but has only 

 recently been applied to plants. A single plant is chosen to be the 

 parent of a new race, and seeds from it are grown in isolation to 

 prevent chance cross-pollination with another plant. From its 

 progeny, in turn, one most desirable individual is chosen; and 

 by this type of selection in a few generations there may result a 

 plant which will produce descendants with very slight variations 

 among themselves. Such pedigree culture succeeds best with 

 self-pollinated plants. This is the method used in preserving 

 mutant characteristics; it has been used with great success in 



