VARIATION AND HEREDITY 13 



to discontinuous variation, and the investigations of 

 Bateson and others, founded on the rediscovered work 

 of Gregor Mendel, abbot of Brtlnn, have brought to 

 light many definite laws. 



As an example of these laws, let us take the case of 

 eye-colour. While many grades are known in the 

 intensity of brown colouration, it is possible to say 

 whether brown pigment is present at all. Its presence 

 and absence are contrasted characteristics. Now the 

 presence of brown pigment in the eye seems to be 

 associated with a definite character in the germ cells of 

 the individual. If that character be present in all the 

 germ cells of either parent, all the offspring will possess 

 brown eyes. If neither parent possesses it, the eyes of 

 none of the children will contain brown pigment, they 

 will all be grey or blue. The brown colour is what is 

 called a dominant character it always shows if it be 

 present. Greyness or blueness is simply due to the 

 absence of brown. If one parent is pure-bred " blue " 

 and the other pure-bred " brown," all the children will 

 be brown-eyed. Hence while brownness is said to be 

 dominant, blueness is said to be "recessive." 



But the brown-eyed children of a mixed couple will 

 not be pure-bred with regard to eye-colour, though, 

 owing to its dominant character, brownness shows in all 

 their eyes. Half their germ cells will carry the brown 

 character, and half not. Hence, by the theory of 

 chances, if two such half-bred individuals mate, one 

 quarter of their children will, on the average, be 

 developed from the union of two " brown " germ cells 

 and be pure-bred " browns," one quarter will be pro- 

 duced by the union of two " blue " cells and be pure- 



