250 Experimental Transformation of Species 



the color formation depends upon the action of a ferment 

 or enzyTue upon a special substance, the chroTnogen, and that 

 each strain had 07te of these factors in its hereditary consti- 

 tution but not the other, there would be no color in the 

 flowers of the pure strains. The hybrid resulting from the 

 crossing, having both factors, produces purple flowers. In 

 the following generation, segregation takes place and the ratio 

 of 9:7 suggests a modified 9:3:3:1, indicative of a hybrid in 

 which two alternative characters are involved (see Fig. 65) . 



In Bateson's experiments, the hybrid purple-flowered 

 strain broke up in the next generation into forms showing 

 seven different color combinations, including the pure white. 

 The breeding tests led to the discovery that there were five 

 distinct factors involved in determining the appearance of 

 the flower. Professor W. E. Castle of Harvard, studying the 

 coat colors of rabbits, found eight different factors involved: 

 and other workers later found a similar series of seven factors 

 related to the production of coat color in mice. 



The use of the concept factors in place of deter7mners 

 has simplified the interpretation of many hybridizations. 

 This idea agrees ( i ) with the fact that in many cases there 

 appear graded series between the two parental types, instead 

 of the sharp separations which Mendel found; (2) with 

 the fact that each variable in the germ affects more than one 

 character; and (3) with the fact that each character of the 

 plant or animal may be influenced by several elements in the 

 germinal plasm. A familiar example of a character that is 

 influenced by several factors is stature. Experiments with 

 wheat and other grains, as well as with animals, show that 

 the distribution of intermediate types into numerous graded 

 groups is in perfect accord with the idea that the trans- 

 mission of this character is determined by several factors. 



The Germ Cells 



In the higher plants and animals the new individual 

 arises from a union of two cells, the Qgg and the sperm. 



