358 TEE POPULAR SCIENCE MONTHLY 



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 classifica- 

 tion. 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 Dioncea 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 Droseracese and those of the gastric juices of animals. The sacs 

 of the aquatic bladder-worts (Utricularia) and the leaves of butter- 

 worts (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 " Ent- 

 deckte 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 



