PREFACE 1x 
Mendelian factors, it would be folly to close one’s 
eyes to so patent a relation. Moreover, as biologists, 
we are interested in heredity not primarily as a mathe- 
matical formulation but rather as a problem concern- 
ing the cell, the egg, and the sperm. 
T. H. M. 
PREFACE TO SECOND EDITION 
We have tried to bring the book up to date not 
only by adding here and there throughout the text 
the latest results on the subject, but also by adding 
two entirely new chapters, and new maps of the 
best known mutant factors. Much new material 
has been added to the chapter on sex; the chapter 
on selection has been largely rewritten. The new 
chapters are one on heredity in Protozoa, and one 
‘on mutation in the evening primrose. In the 
latter field, the latest results of de Vries and others 
on Oenothera, and the work on balanced lethals, 
bid fair to bring the earlier discoveries of de Vries 
into line with more recent work in the whole field 
of mutation and inheritance. 
In place of the “Appendix” we have prepared a 
small manual for laboratory use (Henry Holt and 
Co., Publishers) that gives directions for carrying 
out genetic experiments with the pomace fly. These 
experiments have been picked out as the ones most 
