DISCONTINUOUS VARIATIONS OR MUTATIONS 527 



When a number of generations of offspring are considered, 

 the continuous variations in each generation tend to fluctuate 

 around an average that is common for all the generations. 

 Consequently the selection of continuous variations seldom im- 

 proves the average, and, if any improvement is obtained in this 

 way, it is soon lost when selection is discontinued. The average 

 yield of sugar in Sugar Beets has been improved and is main- 

 tained by selecting for seed the plants having the highest sugar 

 content, but the improvement is lost when selection is discon- 

 tinued. Johannsen has demonstrated that, if one starts with a 

 pure line, that is, with the offspring of a single individual pro- 

 duced by self-fertilization, and keeps the generations pure by 

 preventing cross-pollination, the average of a fluctuating continu- 

 ous variation cannot be increased. He clearly demonstrated this 

 with Beans, which self-pollinate and hence remain pure. He at- 

 tempted to increase the average size of the seeds by selecting 

 the largest seeds of each generation of a certain variety of Beans 

 for planting. He continued this for a number of generations, 

 but obtained no increase in the average size of the seeds. Simi- 

 lar results have been obtained by other investigators in attempt- 

 ing to intensify certain desirable variations in Wheat, Oats, and 

 in pure lines of other plants. For example, an effort to increase 

 the yield in a strain of Oats by selecting each year the best 

 yielding plants for seed gave practically no increase in yield 

 after a number of years of selection. 



Discontinuous Variations or Mutations. That there are 

 two kinds of variations was observed by Darwin and others 

 before and after him, but De Vries was the first to show the 

 importance of discontinuous variations in evolution. He formu- 

 lated the theory that discontinuous variations and not con- 

 tinuous variations furnish the material for natural selection. 

 Discontinuous variations differ from continuous variations in 

 that they arise suddenly, are usually of marked character, and 

 breed true, that is, they are passed on to the offspring. Dis- 

 continuous variations De Vries called mutations. A plant that 

 gives rise to mutations is said to mutate, and a plant arising by 

 mutation is called a mutant. De Vries's theory of evolution is 

 known as the mutation theory, and its fundamental conception 

 is that species are formed by the selection of mutations and not 

 by the selection of continuous variations. 



