DARWIN MEMORIAL CELEBRATION 
31 
less from the parent (variation). The thus modified offspring, subjected 
to natural selection, ultimately perish if they are unadapted, but survive 
if they are adapted to their surroundings. Repetitions of this process 
finally bring the descendants of plants to differ materially from their ancestors 
(evolution). The end of the process seems to be the development of organ¬ 
isms 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 cpiite 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 exqui¬ 
site 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 Droseraeeae and those of the gastric 
juices of animals. The sacs of the aquatic bladder-worts (JJtricularia) 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 proven. 
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 publi- 
