196 PLANT PHYSIOLOGY AND ECOLOGY 



represented by a larger number of individuals, the unfavorable ones 

 by fewer. This process by which competition, the factors of the 

 habitat, or the two acting together, pick out certain plants to the 

 disadvantage of others is called natural selection. Essentially 

 the same thing occurs in artificial selection, when the florist selects 

 for propagation plants that have a desired feature, and neglects 

 or destroys the remainder. Darwin felt, moreover, that favorable 

 variations tended to grow more and more marked with each 

 succeeding generation. As a consequence, the individuals showing 

 these would become more and more numerous and distinctive, 

 while those with other modifications would decrease and finally 

 disappear. After many years, probably after several centuries, 

 a form sufficiently distinct to be called a variety or species would 

 be produced. Forms arising in such a manner are termed variants. 



The critical point in the theory of origin by indefinite variation 

 is the action of natural selection in preserving and accumulating 

 minute differences. The presence of selection in nature is uni- 

 versally recognized. The tendency for a habit to become fixed 

 is generally conceded. Yet it must be stated that there is no 

 experimental proof that natural selection acts in the way assumed 

 by Darwin. Until such evidence is obtained from careful experi- 

 ments, it must remain doubtful w^hether new forms can be produced 

 by variation and natural selection. 



209. Origin by mutation. ]\Iutation takes place when one 

 or more individuals of a form show a sudden and more or less 

 marked departure from it in one or more features. Differences 

 of this sort are extremely rare in nature, at least in comparison 

 with adaptations and variations. Sports are more frequently seen 

 among cultivated plants, probably owing to the intensive action of 

 cultivation. In nature one may expect to find a white-flowered 

 sport of any species with red, blue, or purple flowers, but the most 

 minute search reveals few other mutations. When the latter 

 occur they are ordinarily represented by very few individuals. 

 A new form arising by mutation is termed a mutant. 



A mutant can rarely be traced to the direct action of the 

 habitat. It is probable, however, that it is the result of delayed 

 or latent response to some change in factor, or of a series of 

 responses set up in the plant by a factor. A species may show 

 mutation in any direction, just as is true of variation. In fact, 

 mutation seems to be merely the appearance of variations 



