208 GENERAL BOTANY 



for this is that the forms thus selected remain true in cases where 

 the new race is produced by hybridization. The detection and 

 cultivation, by pedigree-culture methods, of mutations occurring 

 in field and garden crops is. one of the most modern phases of plant 

 improvement and is an important addition to the older methods 

 of mass culture and hybridization. 



EVOLUTION 



The facts and processes relating to the evolution of plants 

 cannot be dealt with at any considerable length in an elementary 

 textbook. The beginning student should understand, however, 

 the close relation which exists between the methods used by man 

 in the improvement of plants outlined in the preceding pages and 

 the process of evolution among wild species in nature. Indeed, 

 Charles Darwin, the greatest of all students and writers on evolu- 

 tion, took the experience arid results of plant and animal breeders 

 as the basis for his theories and investigations in evolution. 



Selection of fluctuating variations. Darwin thought that new 

 species arose in wild forms of life by the gradual accumulation of 

 fluctuating variations, in much the same way as a plant breeder 

 now produces a larger fruit or a more beautiful flower by con- 

 stantly selecting the best plants from those which tend to vary 

 in a given direction. Darwin also endeavored to show that a 

 process of selection takes place in wild nature not unlike that 

 exercised by man, due to the competition and struggle occurring 

 among plants, caused by the overproduction of seeds. " Can it 

 then be thought improbable," says Darwin, " seeing that varia- 

 tions useful to man have occurred, that other variations, useful 

 in some way to each being in the great and complex battle of 

 life, should occur in the course of many successive generations. 

 If such do occur, can we doubt (remembering that many more 

 individuals are born than can possibly survive), that individuals 

 having any advantage, however slight, over others, would have 

 the best chance of surviving and procreating their kind. This 

 preservation of favorable individual differences and variations, 

 and the destruction of those which are injurious, I have called 

 natural selection, or the survival of the fittest " 



