PREFACE 



THE call for a revised edition of The Theory of the 

 Gene has given me an opportunity to make several 

 corrections of the original text as well as to bring 

 into closer relation the references in the text to those in 

 the bibliography. The bibliography has undergone a care- 

 ful revision, a number of omissions have been made good, 

 and a number of new references added. 



In the same year in which the first edition of the Theory 

 of the Gene appeared, I published a short article in the 

 Quarterly Review of Biology in which certain questions 

 relating to sex and fertilization were discussed. This topic 

 was not taken up in the first edition, although intimately 

 related to the subjects there considered. The sexual un- 

 ion of the plus and minus strains of certain fungi and 

 algae, in relation to the fertilization of the eggs of the 

 higher plants, raises a number of questions of fundamen- 

 tal interest to biologists. With the permission of the pub- 

 lishers, Williams, Wilkins, and Co., I have reprinted here 

 those parts of the original article that bear on the ques- 

 tion, as an extension of the chapter on "Other Methods of 

 Sex-Determination involving Sex Chromosomes." 



During the last two years many papers have appeared 

 dealing with the number, and changes in number, of the 

 chromosomes. It has not been possible to incorporate these 

 new contributions, nor is it necessary, since, for the most 



