thrown on the origin of man and his history." It would have impeded the
acceptance of the theory of natural selection if Darwin had paraded,
without giving any evidence, his conviction with respect to man's
origin. When he found, however, that many naturalists accepted his
doctrine of the evolution of species, it seemed to him advisable to work
up such notes as he possessed, and to publish a special treatise on the
origin of man. He was the more glad to do so, as it gave him an
opportunity of discussing at length sexual selection, a subject which
had always interested him.

Darwin's book on the "Expression of Emotion in Men and Animals" was
published in the autumn of 1872. This had been intended to form a
chapter on the subject in the "Descent of Man," but as soon as Darwin
began to put his notes together he saw that it would require a separate
treatise. In July, 1875, appeared the book on "Insectivorous Plants."
The fact that a plant should secrete, when properly excited, a fluid
containing an acid and ferment closely analogous to the digestive fluid
of an animal, was certainly a remarkable discovery. In the autumn of
1876 appeared "The Effects of Cross and Self Fertilization," a work in
which are described the endless and wonderful contrivances for the
transportation of pollen from one plant to another of the same species.
About the same time was brought out an enlarged edition of the
"Fertilization of Orchids," originally published in 1862. Among the
minor works issued during the later years of Darwin's life may be
mentioned particularly the little book on "The Formation of Vegetable
Mould through the Action of Worms." This was the outgrowth of a short
paper read before the Geological Society more than fourteen
years before.

In order to appreciate the enormous amount of research accomplished by
Charles Darwin, it is needful to keep in mind the conditions of
ill-health under which almost continually he worked. For nearly forty
years he never knew one day of the health of ordinary men. His life was
one long struggle against the weariness and drain of sickness. During
his last ten years there were signs of amendment in several particulars,
but a loss of physical vigor was apparent. Writing to a friend in 1881,
he complained that he no longer had the heart or strength to begin any
prolonged investigations. In February and March, 1882, he frequently
experienced attacks of pain in the region of the heart, attended with