injurious. The phenomena do not now, however, appear to have as important a relation to evolution as they were formerly supposed to have, and Darwin later expressed regret that he had not given more attention to the processes of self-fertilization.
His interest in showing that cross-fertilization was beneficial led him to closely investigate the various structural features of flowers which necessitate this process to a greater or less degree, such as diœcism, monœcism, polygamy and heterostyly; his observations and speculations are presented in the volume entitled "Different Forms of Flowers and Plants of the Same Species," published in 1877. He records that making out the meaning of heterostyled flowers gave him very great pleasure. A chapter of the book is devoted to cleistogamic flowers, which are necessarily self-fertilized and produce seed abundantly. This work is largely a revision and rearrangement of several papers previously published in the Journal of the Linnæan Society.
"The Variation of Animals and Plants under Domestication," Darwin's largest work, appeared in 1868, published in two volumes. As bearing on this topic he had studied, among plants for many years, the cereal grains, garden vegetables, edible fruits, ornamental trees and ornamental flowers. In the preface he again discusses natural selection and defines it as "This preservation, during the battle for life of varieties which possess any advantage in structure, constitution or instinct," noting that Herbert Spencer had well termed the same process "The Survival of the Fittest." But the bulk of the work is given to the consideration of selection by man—artificial selection, by which races useful to us, economically or esthetically, have been preserved and modified, some of them having originated in very remote times and been taken advantage of by uncivilized man. A chapter is devoted to the phenomena of bud-variation, in which many cases of branches of plants different in one respect or another from other branches on the same plant are described in detail. Many of these have been taken advantage of by horticulturists for the propagation of valuable races. He did not reach any definite conclusion as to the cause of these interesting occurrences; but recently acquired knowledge of mutation seems to indicate that they are of that category, differing from seminal mutations in that a cell in the axil of a leaf is affected rather than a germ-cell. In these volumes we find Darwin's most detailed discussion of heredity of variability and of hybridism and the last chapter outlines his provisional hypothesis of Pangenesis, an ingenious supposition, applying to living matter the general features of the atomic theory, with an additional inherent power of reproduction of the atoms or "gemmules" as he termed the hypothetical ultimate particles.
The movements of plants and of their various organs were also studied by Darwin for many years. His first essay on this topic
