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. 
