THE constantly increasing interest in all phases of evolution has made
necessary the preparation of a second edition of this book within a few
months after the first appeared. The opportunity has been used to
eliminate typographical errors, and to make alterations in the form of a
few sentences for the sake of clearness and smoothness. The subject
matter remains practically unchanged. An explanatory note has been added
on page 575 in order to avoid confusion as to the identity of some of
the plants which figure prominently in the experimental investigations
in Amsterdam and New York.
The portrait which forms the frontispiece is a reproduction of a
photograph taken by Professor F.E. Lloyd and Dr. W.A. Cannon during the
visit of Professor de Vries at the Desert Botanical Laboratory of the
Carnegie Institution, at Tucson, Arizona, in June, 1904.
D. T. MACDOUGAL.
December 15, 1905.