312 EVOLUTION, GENETICS, AND EUGENICS 



MUTATIONS 



Mutations are, by definition, changes in the germinal substance. 

 They may affect individual genes or whole bundles of genes (chromo- 

 somes). They may occur in the germ track or in the soma. If they 

 occur in the soma, the resulting change is seen in a local patch of 

 changed cells, the size of which depends on whether the mutation oc- 

 curred early or late in ontogeny. Since body cells do not produce germ 

 cells, somatic mutations are no more heritable than are functional 

 changes in the soma. The only mutations of significance for evolution 

 are those that occur in the germ track. The various kinds of germinal 

 mutations will be discussed at some length in the following chapter. 



