10 BREEDING CROP PLANTS 



1. Variability. It is a matter of common observation that 

 no two individuals are exactly alike. If sufficient individuals 

 are examined the range of variation is found to be quite great. 

 These variations are universally present. 



2. A Struggle for Existence. If all the progeny of some of the 

 lower forms grew to maturity and each in turn produced as 

 many progeny, the world would soon be overrun with a single 

 form. There is competition also between different species and 

 genera. 



3. Natural Selection. The conclusion would certainly seem 

 reasonable that those forms would survive which possessed 

 characters better adapted to a given environment and there- 

 fore gave those particular forms advantage in the struggle for 

 existence. 



4. Heredity. Variation produces the material for natural 

 selection to work upon and heredity tends to perpetuate the 

 variations. 



The mechanism of transmission of characters, the physio- 

 logical cause of variations, and the inheritance of different- 

 categories of variations were unanswered problems. Many 

 criticisms were made of Darwin's work and many were considered 

 by Darwin himself. Nearly all of these have a bearing upon 

 plant breeding. In the improvement of crops, artificial selection 

 takes the place of natural selection. The breeder is constantly 

 faced with the question of the perpetuation of a variation. He 

 also faces the question of whether the useful variation will per- 

 petuate itself in crosses or will be lost. 



Darwin recognized two sorts of variations, the " fortuitous" 

 or chance variations, i.e., those which are everywhere present and 

 which cause every plant to be slightly different from other plants 

 of the same species. These were considered to be of primary 

 importance in evolution. While he recognized "definite" or 

 discontinuous variations, the so-called mutations, these were not 

 considered of primary importance. 



The Stability of the Germ Plasm. Weissmann's theories are 

 of much interest. He developed the idea of the continuity of the 

 germ plasm and that external agencies could not modify 'inheri- 

 tance without first affecting the germ cells. Plant breeders are 

 not particularly interested in Weissmann's ingenious theories 

 which were outlined to show that the inheritance of acquired 

 characters was an impossibility. Apparently, in order that a new 



