216 GENETICS AND EUGENICS 



of mutation within a pure line of beans. That such mutations 

 must occasionally occur or at least have occurred in times 

 past is shown by the very existence of races differing in 

 genetic constitution. By crossing these we can produce in- 

 termediates of any desired size. This shows that the genetic 

 differences between them are numerous (on the multiple 

 factor h;^^othesis) and numerous genetic differences have, 

 most probably, not originated at one time or place. Studies 

 of other self -fertilizing plants, such as peas, oats, wheat and 

 tobacco, support the view that genetic variations in such 

 species are rare as compared with the variability to be se- 

 cured by artificially crossing different varieties, in which the 

 beneficial genetic changes of centuries may have accumu- 

 lated. All these are immediately made available for recom- 

 bining in every possible way with the genetic variations ac- 

 cumulated in any other variety, when the two are artificially 

 crossed. Any advantageous genetic variations which have 

 made their appearance in a self-fertilizing plant, from the 

 time it was taken into cultivation to the present time, are 

 likely to be found in varieties now in cultivation, since if 

 such variations had survival value they would naturally in- 

 crease and come to predominate in the crops of successive 

 years, even if no conscious selection was exercised. Acci- 

 dental cross-pollinations, such as are known to occur occa- 

 sionally in any species normally self-fertilized, would give 

 opportunity for combination to arise of two or more advan- 

 tageous genetic variations, distinct in origin. Subsequent 

 self-fertilization for ten or more generations would establish 

 in homozygous lines all possible combinations of the genetic 

 factors introduced in the accidental cross. Thus it happens 

 that a field crop of any seK-fertilizing plant contains a great 

 number of pure lines, each considered by itself a pure-breed- 

 ing homozygous variety. In such cases the work of the plant 

 breeder is very simple. He has only to isolate the varieties 

 which nature gives him ready-made and test these out to 

 determine which can most profitably be grown in a particu- 

 lar region or under a particular set of field conditions. In 



