CHAPTER I 



Introductory. Structure of the Germ Plasm. Genetic 

 Variations Resulting from New Combinations of Genes 



EXPERIMENTAL genetic science has said of late, with 

 some justification, that "the problem of heredity is 

 solved" ; solved in principle, though not in all its details. It is 

 solved to the extent that the nature of the processes which 

 underlie it are largely known; and so far as not known, they 

 are localized and open for study. 



Can experimental genetic science say that the problem of 

 evolution is likewise in principle solved? Are the processes 

 which underlie evolutionary change known ? This question 

 forms the theme of the present study. 



The processes which underlie organic evolution are bound 

 to be genetic variations of some sort; that is, changes in the 

 inherited characteristics. Experimental genetic science is now 

 able to detect genetic variations as they occur in living organ- 

 isms. Single genetic variations are isolated, the precise time 

 and place of their occurrence determined; they are subjected 

 to minute study in the laboratory. Within the last few years 

 men have learned how to produce these genetic variations; 

 this is perhaps the most important condition for determining 

 the nature of such changes. In a thousand laboratories men 

 are at work on these things ; genetic variation is now one of 

 the most active subjects in biological science. The observed 

 genetic variations are of a number of diverse types. Do these 

 known types furnish the material of evolution ? Are the single 

 variations steps in evolution ? Does genetic science therefore 



