PREFACE 



In 1928 the authors of this work commenced to collect and 

 arrange data on the variation of animals in Nature. Any 

 naturalist, particularly the systematist and the student of 

 geographical distribution, will realise that there are many 

 methods and subjects of inquiry which might be usefully 

 adopted in analysing the vast amount of detail which has 

 accumulated on this subject. We felt, however, after some 

 time that we could make our analysis most useful if we tried 

 to show the relation between natural variation and the main 

 problem of the causes of evolution. We came to the conclusion 

 that, in spite of the many valuable contributions to this 

 subject, a review which was both synthetic and critical was 

 still necessary. The subject has become so complex of recent 

 years, so many special lines of research have been opened up 

 and the accumulated literature relevant to the subject has 

 become so intractable, that a synthesis of the sort we have 

 attempted is an urgent necessity. To exemplify the need for 

 such a synthesis we would point out that of the observations, 

 experiments and theories made by workers a generation or more 

 aso some have become the matter of text-books and current 

 biological teaching, some have been neglected and forgotten, 

 and others again are still the subject of ill-informed controversy. 

 There is a great need for an overhaul of our heritage of research 

 and observation and for an exact valuation of much that is 

 either summarily neglected or accepted without question or 

 scrutiny of the original publications. 



We do not claim that in this work we have produced either 

 an exhaustive survey or a novel viewpoint which might illu- 

 minate an old and contentious problem. The method we 

 have adopted differs very little from that elaborated by 

 Darwin, though we have tried to formulate the problem in 

 accordance with the many generally accepted changes which 



