220 MENDELISM 



possibly by a definite substance or combination of sub- 

 stances. The pair of parental factors for a particular 

 character would combine on fertilization, and at the 

 formation of the gametes in the offspring its members 

 would separate as perfectly definite entities, to re- 

 combine when these gametes meet once more with their 

 corresponding mates. Such a definite segregation of 

 characters taking place within a pure strain would be 

 very difficult of absolute demonstration, but it is hard to 

 avoid the conclusion that this is a true deduction from 

 the facts observed when cross-breeding takes place. 

 Such a segregation would formerly have been thought a 

 very small assumption in comparison with that of the 

 segregation of pairs of allelomorphs of which no trace 

 is externally visible, and yet the latter assumption 

 has now been shown to be perfectly well established. 



This idea of unit characters, capable of being inherited 

 independently of one another, is one of the most 

 important conceptions which has ever been introduced 

 into the science of biology, and the introduction of it 

 has followed as the direct result of Mendel's work. It 

 is a conception which has led to a complete change in 

 our ideas of heredity, since we no longer look upon 

 the individual as a unit, but find ourselves compelled 

 to study separately the independent characters of 

 which the individual is built up. The idea of the 

 individual as a living mosaic an idea put forward long 

 ago by Naudin with only a partial realization of its 

 significance has thus returned to us. In this con- 

 nection a curious problem presents itself. What 

 would be left if we could imagine all the separable 



