PREFACE TO THE THIRD EDITION 
In this edition it has been found necessary to make a 
further correction of the account of the yellow coat 
colour in mice. Problems connected with the yellow 
colour still remain unsolved in more than one group 
of animals. 
I have taken the opportunity of introducing into 
the concluding chapter a brief statement of the recent 
discoveries relating to so-called-‘ graft-hybrids.’ These 
probably represent the most important advance in our 
knowledge of the hereditary processes of plants which 
has been made within the past two years. 
Other corrections and alterations are of minor 
importance. 
R. H. LOCK. 
PERADENIYA, CEYLON, 
May 17, 1911. 
PREFACE TO THE SECOND EDITION 
Tue following are the principal alterations and addi- 
tions which have been made in the present issue : 
A short list of references has been added at the end 
of each chapter. These include only a small number 
of works which may be regarded as essential for every 
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