PREFACE 



Few biologists will doubt that heredity, the subject matter of genetics, 

 is one of the important problems of biology. Indeed, when Mendel's 

 results were rediscovered in 1900 and the new science of genetics got 

 under way, it appeared to some authors that a new era was dawning 

 in biological thought. But in the years that followed it often seemed 

 that the high hopes which had been entertained were becoming dissi- 

 pated in a morass of numerical elaborations on the hackneyed theme 

 of MendeHan segregation. Genetics was in danger of being considered, 

 by other biologists, as a world of its own, devoted to following the 

 comings and goings of genes whose relevance to other biological 

 phenomena, though incontrovertible in general theory, could rarely be 

 stated in detail and in particular. 



I beheve such an impression to be quite false. Experimental breeding, 

 and the determination of the ratios between classes of offspring, is no 

 more than one of the main techniques of genetics; and an over-emphasis 

 on technique nearly always obscures the real interest of a science, 

 which lies in the concepts and theories to which the experimental 

 methods open the door. There was inevitably a period when geneti- 

 cists had to concentrate their efforts on putting the basic ideas of their 

 science on a firm foundation. In recent years genetics has been able 

 to apply itself to wider problems, and it has produced results which 

 cannot be overlooked by any student of evolution, development or 

 cytology. 



In this book I have tried to give an account of these recent develop- 

 ments. The literature of genetics has become extremely large, probably 

 larger than that of any other branch of experimental biology except 

 medical physiology, and it is impossible to do more than provide some 

 help to the student who wishes to plunge into this sea of material. The 

 bibUography is therefore rather extensive although, of course, not 

 exhaustive. The aim has been to cite summaries, reviews and articles 

 in which general questions are discussed and ftirther references given, 

 although I am conscious that this may sometimes appear to do injustice 

 to the older authors on whose work the present-day edifice is erected. 



I should add a word of explanation, and perhaps of apology, for 

 having written a text-book of genetics although the field of my own 

 research is usually classified as the separate science of experimental 

 embryology. Perhaps the objea of the book, as I have explained it 



