Selection of Elementary Species 99 



In order to get a closer insight into the 

 causes of this confused condition of ordinary 

 races, Rimpau made some observations on 

 Rivett 's wheat. He found that it suffers from 

 frost during winter more than the local Ger- 

 man varieties, and that from various causes, 

 alien seeds may accidentally, and not rarely, 

 become mixed with it. The threshing-machines 

 are not always as clean as they should be and 

 may be the cause of an accidental mixture. 

 The manure comes from stables, where straw 

 and the dust from many varieties are thrown 

 together, and consequently living kernels 

 may become mixed with the dung. Such stray 

 grains will easily germinate in the fields, where 

 they find more congenial conditions than does 

 the improved variety. If winter arrives and 

 kills quantities of this latter, the accidental local 

 races will find ample space to develop. Once 

 started, they will be able to multiply so rapidly, 

 that in one or two following generations they 

 will constitute a very considerable portion of 

 the whole harvest. In this way the awnless 

 German wheat often prevails over the intro- 

 duced English variety, if the latter is not kept 

 pure by continuous selection. 



The Swiss wheat-breeder Risler made an ex- 

 periment which goes to prove the certainty of 

 the explanation given by Rimpau. He ob- 



