4 Introduction 



other border fields. Since the present work is neither a textbook nor 

 a review, it is assumed that the reader has advanced knowledge of 

 genetics as well as of general biology.^ 



It is obvious that some more or less arbitrary decisions have had 

 to be made in regard to the inclusion of topics. To include every topic 

 on which theoretical discussions are possible or have been made would 

 be a hopeless undertaking. I tried to make it a rule not to enter in 

 detail into problems which, though interesting in themselves, do not 

 contribute much to the general theory of genetics. Hence, for example, 

 the theory of crossing over is mentioned only briefly because it is con- 

 sidered to be a special problem. For general genetic theory it is very 

 important that crossing over occurs. But whether Janssens' or Belling's 

 or Darlington's or any other theory is correct does not affect the theory 

 of the gene and its action. The aspects of this problem that bear upon 

 general theory will be mentioned, but the specific theories will not 

 be taken up. This does not mean, however, that one day a new solu- 

 tion of the problem may change this situation completely and make 

 it focal for genetic theory. This type of limitation will occur with 

 many individual topics, and I suggest that a reader who misses a topic 

 in which he is interested ask himself whether I did not omit it for the 

 reason indicated above. In a number of cases he will find that I dis- 

 cussed the topic in my other writings, but decided that it does not 

 belong here. Of course, another author would probably have different 

 ideas and make different selections. 



