30 EVOLUTION OF LIVING ORGANISMS. 



lias been transmitted to it ; yet it may inherit factors 

 which remain undeveloped, and the presence of which 

 may be revealed by their transmission to and effects on 

 later generations. Now when two races are crossed, 

 such as the tall and dwarf peas mentioned above, the two 

 opposed or allelomorphic characters, as Bateson calls 

 them, may be due not to the result of two different 

 factors, in the one case representing tallness and in the 

 other shortness, but merely to the presence in the first of 

 a factor for tallness which is absent in the second. Thus 

 a pure recessive would be without a factor present in the 

 pure dominant and in the hybrid.* Moreover an appar- 

 ently simple character may be due not to a single factor, 

 but to the co-operation of several, which must all be 

 present at the same time and in the same zygote for the 

 character to appear. Yet they are sufficiently independ- 

 ent to be capable of segregation, and to give rise each to 

 some different character. Thus the colour of an animal 

 or plant may be a complex character due to a group of 

 factors transmitted as a whole, and so far constant in the 

 species, but which can be analysed out into a number of 

 separate strains, each breeding true to its own new 

 colour. The grey colour of the wild mouse, for example, 

 has been shown to depend on the co-operation of at least 

 six different factors. Change any one of them and the 

 resulting colour will be changed also. A colour factor 

 may be unable to show itself in the resulting character, 

 unless accompanied in the zygote by an independent 

 colour-developing factor. Thus a white individual may 

 hold colour factors, but appear as an albino because it 

 lacks the essential colour-developing factor ; and there 

 may be as many different varieties of partial albinos as 

 there are colour factors in the species capable of segrega- 

 tion. If crossed they will always breed true to albinism, 



* This "Presence and Absence theory" is not universally accepted, 

 many authors believing that both allelomorphic characters are repre- 

 sented in the germ-plasm by factors. 



