14 GENETIC VARIATIONS 



since the number of types and modifications is very great, the 

 number of diverse combinations of characteristics that may 

 occur is practically unlimited. 



Observe now the relation of all this to genetic variations. 

 The genetic system presents three diverse aspects, each of 

 which may be altered; each of the three yields a distinctive 

 type of genetic variation. First, the genetic system is com- 

 posed of many diverse organic materials, the genes. These 

 materials might be altered, chemically or otherwise, giving 

 rise to new characteristics; this would be the fundamental 

 type of genetic variation. The genes are arranged in a definite 

 structure, the genetic system. This structure may be altered; 

 this gives a second type of genetic variation. Third, within 

 the framework of this structure, there are diverse combina- 

 tions of genes, in different individuals. Changing the combi- 

 nations of genes that are present gives a third type of genetic 

 variation. 



Genetic variations of all these types are known, or at least 

 are commonly held to be known; they have been described in 

 detail. About one of the three types, however, serious doubt 

 has recently arisen; this is a matter with which we shall deal 

 later (see Chapter III). 



We shall take up these three types of genetic variation in 

 the reverse order from that in which I have named them. We 

 shall take up first, changes in the combinations of genes 

 present; then changes in the structure of the genetic system, 

 reserving the supposed changes in the material of the genes 

 for our third topic. Beyond this I plan to present certain other 

 genetic variations not precisely fitting into this classification. 

 In the limits of these pages I shall have to confine myself to 

 types of variation observed to occur in living organisms. We 

 shall look at each type in relation to evolution. 



