222 BRITISH PLANTS 



existing races. Most of our agricultural crops are aggre- 

 gate races containing an enormous number of elementary 

 species mixed together. An ordinary sample of seeds 

 may contain several. If these are sown, and among the 

 plants produced one is seen to be conspicuous by char- 

 acters which are likely to increase its value as a crop, 

 this individual is isolated ; its seed is carefully collected 

 and sown apart, and very soon a new variety is placed 

 on the market. In this case, man puts Nature through 

 a sieve, and selects out for cultivation those forms which 

 he considers most valuable. 



