PREFACE TO THE FOURTH EDITION 



the fourth edition and for making those alterations 

 and valuable additions which the lapse of time rendered 

 necessary. I cannot put into adequate words my 

 appreciation of this proof of the friendship which 

 existed between Dr. Doncaster and my husband, j 

 am also indebted to Professor Bateson, F.R.S., for his 

 valued advice on Millardet's hybrids. 



B. L. 



NOTE TO FIFTH EDITION 



IN revising the book for a fifth edition, I have made 

 only slight alterations. A few sentences and footnotes, 

 referring to work published since 1916, have been 

 added; as in the fourth edition, these are enclosed 

 in square brackets. The only other alteration that 

 requires mention is the omission, in several places, of 

 the words ' recent,' ' recently published,' or others of 

 similar import, when they referred to events or ideas 

 now some fifteen or more years old, and of expressions 

 of doubt about facts which have subsequently been 

 definitely established. With the exception of these 

 slight textual alterations, made advisable by the lapse 

 of time, and of the passages enclosed in brackets, the 

 book remains substantially as it was left by the author. 



L. D. 



Jan. 1920. 



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