viii Preface 



Chapters 24 through 30, the fourth and last part of the book, 

 deal with what are frequently called "chromosomal aberrations." 

 If we accept an ideal concept of chromosomal behavior during 

 cell division and reproduction and if we accept the 2n number 

 as the ideal number of chromosomes in the animal soma or in 

 the plant sporophyte and the n number as ideal in the plant 

 gametophyte, any departure from these ideal conditions repre- 

 sents an aberration. The various types of aberrations are de- 

 scribed in this section, and their bearing on problems of evolution 

 is discussed. This material is often called "cytogenetics," al- 

 though any correlation at all between genetic data and cytologi- 

 cal observations should properly bear this designation. 



Throughout I have tried to avoid being dogmatic on all or 

 most controversial issues. Sometimes I have attempted to pre- 

 sent all the important theories concerned in the explanation of 

 certain data without expressing any preference, and on some 

 points where I have favored one theory I have presented other 

 theories for the student to consider. 



Because of its scope, I have had to restrict the bibliography 

 somewhat. Many important papers have had to be omitted en- 

 tirely and where an author had published a series of papers on 

 the same subject, I have listed only a few. Although I did not 

 adhere rigidly to any rule, I frequently listed the first paper of 

 the series and the most recent. I usually, also, included papers 

 that contained extensive bibliographies or summarized informa- 

 tion and those that were especially outstanding for the theories 

 or conclusions that they presented. Even though a paper was 

 referred to in more than one chapter, I included it in the bibli- 

 ography only once. 



Several persons have read all or part of the manuscript, and 

 to them I wish to express my deepest appreciation. However, 

 I must emphasize that they are in no way responsible for any 

 of the errors that may appear in the book. Professor George 

 H. Shull of Princeton University has read and criticized the en- 

 tire text in manuscript, and I am very grateful to him for many 

 suggestions. I also wish to thank Professor P. W. Whiting of 

 the University of Pennsylvania for his kindness in reading and 

 criticizing the manuscript of parts of Chapters 16 and 29. 

 Doctor Alexander Wiener of Brooklyn, New York, read the 

 manuscript of most of Chapter 19 and made many suggestions 



