8 ELEMENTS OF AGRICULTURE 



and eventually kill them. These will in turn raise seed, 

 and the process will be repeated. Thus, the weak, the ill- 

 adapted, are always being eliminated. This constant se- 

 lection of the strongest, the ones adapted to the conditions 

 under which they are to live, may eventually result in a 

 changed type. There are countless examples of changes 

 that were probably brought about in this way. 



10. Sports, or Mutations. Sometimes a plant appears 

 that is quite unlike its brothers. Usually, such a one does 

 not have the power to transmit its qualities to succeeding 

 generations. The offspring revert to the former type. 

 Such a new form is also likely to be poorly adapted to the 

 environment, so that it is quickly exterminated. But, 

 occasionally, a sport may occur that is better adapted 

 to the environment than the type from which it came, 

 and that also has the power to impress its characters on 

 its offspring. In such a case, it will crowd the old form 

 out and give a new type in its place. Such a change might 

 be rather rapid as compared with the ones produced by 

 natural selection alone. The polled Shorthorn and Here- 

 ford cattle are sports. Usually such sports are lost by breed- 

 ing with the common type. 



11. The Development of Weeds by Natural Selection. 

 Our common weeds are a good example of the adaptation 

 of plants to particular conditions. Nearly all of our bad 

 weeds are natives of Europe. For centuries they have been 

 growing in the cultivated fields, until each has developed 

 certain characteristics that have enabled it to persist in 

 competition with the crops and in spite of man's efforts 

 to subdue it. Only a few of the native American plants 

 are able to persist in cultivated fields; but these foreigners, 



