Introduction 3 



theory or theoretical genetics. In my first book of this kind (1920a), 

 which is probably unknown to present-day geneticists, I started with 

 the facts derived from the discovery of intersexuality and its genetical 

 and developmental analysis, which led me to present definite ideas 

 about the gene and its action, and to bring together genetics, develop- 

 ment, and evolution on the basis of a simple generalization. Ever since, 

 I have felt constrained to review the accumulating facts, my own as 

 well as those of other investigators, with the intention of building up 

 a general theory of heredity which would unify and explain the welter 

 of facts. Thus, while my experimental, factual work remained always 

 in the foreground I indulged from time to time in the dangerous 

 pleasure of looking over the whole and trying to explore the possi- 

 bilities for broad general conceptions. The major steps in this develop- 

 ment were my books of 1920a, 1927, 1938a, and 1940, and such general 

 discussions as those of 1934^7, 1938d, 1944, 1946^?, 1948c, 1951a, 1952a, 

 and 1954. 



Sewall Wright's work approached the same goal during the same 

 time, and many points of contact as well as divergence exist. 



I consider the time ripe for a review of the basic features of 

 genetics so far as they contribute to the emergence of a theoretical 

 genetics. It is not my intention to present a complete review and dis- 

 cussion of the facts and all the ideas which have been oflFered to ex- 

 plain them. I propose, rather, to select the salient facts and ideas, and 

 to present them as I see them in their meaning for the general theory 

 of genetics. Hence this study will not be a text or a review, but rather 

 an intimate and personal report, a dialogue with others of a similar 

 or a different mental attitude, a dialogue in which (as is true of all 

 good dialogues) I endeavor to convince my interlocutors of the cor- 

 rectness of my own point of view. But I realize that it is rather unim- 

 portant whether I succeed in this or not, so long as I and perhaps a 

 few others may derive satisfaction from a well-rounded synthesis. 



Theoretical genetics comprises all the problems connected with 

 the following questions: (1) What is the nature of the genetic mate- 

 rial? (2) How does the genetic material act in controlling specific 

 development? (3) How do the nature and action of the genetic ma- 

 terial account for evolution? These three major problems of genetics 

 are closely interconnected, for the answer to one affects the answer 

 to the others. Since these problems cover the entire body of genetical 

 facts, their analysis will have to touch upon numerous individual 

 phases of genetics, including the disciplines upon which genetics 

 rests, such as cytology and experimental embryology, but also many 



