vi PREFACE TO THE SECOND EDITION 
serious student of genetics. For a fuller bibliography 
reference must be made elsewhere (see end of 
Chapter I.). 
The account of the evidences of evolution in 
Chapter II. has been somewhat expanded. 
In Chapter III. a brief note on Isolation has been 
inserted, and the remarks on Mimicry have been 
modified. 
In Chapter IV. a definition of the Co-efficient of 
Variability has been added, and a more definite attitude 
has been adopted with regard to the theory of the 
pure line. 
To Chapter V. there have been added an account of 
the effect of external conditions upon plants and a 
brief allusion to the artificial production of muta- 
tions. 
The most important changes in Chapter VIII. are 
as follows : 
An account of partial gametic coupling has been 
added ; the description of the inheritance of coat colour 
in mice has been revised ; a classification of the different 
forms of latency has been inserted. Further additions 
are a discussion of the case of supposed non-segregating 
hybrids, and some remarks on purity of strain. 
The problem of sex determination has been further 
discussed in Chapter IX. 
Chapter X. as a whole is new. 
R. H. LOCK. 
PERADENIYA, CEYLON, 
May 17, 1909. 
