iv PREFACE 



tions are likely to include names from Pacific lands also. 

 For advance in science never results merely from brilliant 

 guesses by the few, but takes place chiefly through the 

 patient, persistent efforts of numerous workers who test by 

 observation and experiment every suggested explanation of 

 the phenomena of nature. This is a task of such magnitude 

 and such importance that in it the cooperation of all nations 

 is needed and fortunately is not withheld. To promote the 

 common good of all is the greatest honor of each. 



The author has found that interest in the subject of 

 heredity is not confined to college classes but is shared by 

 people of intelligence everywhere, because it touches and 

 affects the lives of all. The animal breeder and the plant 

 breeder have an intensified interest in the subject because it 

 vitally concerns the success or failure of their occupations. 

 The needs of this wider public have been kept in mind in the 

 preparation of this book, but it has not been thought neces- 

 sary to omit on this account discussion of questions re- 

 quiring thoughtful consideration for their full understanding. 

 A discussion which evokes no independent thinking, or even 

 opposition, is not likely to extend knowledge, the teacher's 

 prime concern. 



1 am indebted to many friends and fellow biologists for 

 assistance in connection with the illustrations, acknowl- 

 edged in the legends of the figures, to Professor B. M. Davis 

 for a critical revision of Chapter VI, and to Professor J. A. 

 Detlefsen for assistance in revising the proofs. My best 

 thanks are due to the publishers who have spared no effort 

 to make their part of the work successful. 



W. E. Castle. 



Cambridge, Massachusetts, 

 December, 1916. 



