THE INDIVIDUAL AND THE SPECIES 197 



children of a brown-eyed mother and a blue-eyed 

 father do not usually have eyes in which the colours 

 of the parental eyes are blended : they are blue-eyed 

 or brown- eyed. The contrasting characters are spoken 

 of as dominant and recessive : if tallness is trans- 

 mitted to offspring, which may nevertheless produce 

 dwarf offspring, the latter character is said to be 

 recessive to tallness. The contrasting characters of 

 the parents therefore remain distinct in the progeny, 

 some of the latter exhibiting the one character and 

 some the other ; while it may happen that the one 

 character or the other may be segregated, so that it 

 only appears in, and is transmitted by, the offspring. 

 There are numerical relationships between the numbers 

 of the offspring in which the contrasting characters 

 appear. 



Obviously, tallness and dwarf ness are not charac- 

 ters which differ in quality : they are different degrees 

 of the same thing. Brown eyes and blue eyes are not 

 necessarily different in quality, for we may think of 

 the same kinds of pigment as being present in the iris 

 but mixed in different proportions. But the term- 

 inology of this branch of biology appears to sug- 

 gest that the contrasting characters are, each of them, 

 something quite different from the other : there 

 are "factors" for "tallness," " dwarf ness," for blue 

 eyes and brown eyes, and so on. These qualities are 

 called " unit-characters," and they are supposed to 

 possess much the same individuality in the germ- 

 plasm as the " radicles " of the chemist possess in a 

 compound. Sodium chloride, for instance, is not a 

 blend of sodium and chlorine : the two kinds of atoms 

 do not fuse together but are held together merely. 

 The analogy is, however, very imperfect, for in the 

 chemical molecule the characters are not those of either 



