process seems to be the development of organisms which are little or not at all subject to variation (monotypic genera). All genera of plants containing a large number of species are evidently subject to continued variation and their species and races almost defy classification. Just what part the phenomena of hybridism take in the final result is not clear, but it may be pointed out that they are evidently unnecessary, because great groups, whole orders, in fact, of the fungi, are devoid of sexuality, and hybridism is therefore impossible among them; yet they are subject to variation like other plants and are quite as difficult to classify.
Observations on insectivorous plants occupied Darwin at intervals from 1860 until the publication of his volume on that subject in 1875. He commenced with the round-leaved sundew (Drosera rotundifolia) while staying at Ashdown Forest, and was soon intensely interested in the exquisite sensitiveness of the leaf-glands to nitrogenous substances. His studies were continued over most of the plants of the sundew family, and to others known to entrap insects or other small animals. He discovered that the leaves of Drosera and of Dionæa secreted a ferment when supplied with various kinds of nitrogenous food and he closely observed the movements of their glands and tentacles and recorded them in detail. Experiments were also made on these plants with a great variety of non-nitrogenous substances. Darwin pointed out the remarkable parallelism between the digestive powers of the secretions of the Droseraceæe and those of the gastric juices of animals. The sacs of the aquatic bladder-worts (Utricularia) and the leaves of butterworts (Pinguicula) were also closely studied. His book is replete with records of careful observations and ingenious deductions. Nepenthes had already been shown by Dr. Hooker to secrete digestive fluids in its pitcher-like leaves, and Sarracenia was suspected of similar activity by Darwin and by others, although he did not regard this as proved.
As early as 1838 or 1839 Darwin was attracted to observe the processes of pollination and noticed the dimorphic flowers of Linum flavum. He had concluded at that time that cross-fertilization was potent in holding species stable and constant. He obtained a great deal of information on this topic in 1841 by reading Sprengle's "Entdeckte Geheimniss der Natur," which stimulated him to continued investigations during summers and he became especially interested in the methods of pollination of the wild orchids growing about his home. This study of pollination of orchids resulted in the publication, in 1862, of his book on that subject, and in it his detailed observations are recorded. Some of his closest observational work was done on this subject of cross-pollination, and he examined a great many species and grew thousands of plants from seed, reaching the broad generalization that cross-fertilization is beneficial to a species and self-fertilization is
