510 The American Naturalist. [June, 
viduals sacrificed, but it is, nevertheless, a great advantage 
which it gains. This may be called mutualism. But there is 
a case of mutualism of plant and animal much more closely 
resembling the mutualism of parasite and host in the vege- 
table world. The mutual inter-dependence of Yucca and a 
moth of the genus Pronuba, is probably the most unique and 
interesting case of mutualism to be found anywhere. This is 
well described by Mr. Webber in the AMERICAN NATURALIST 
for September, 1892. In this case the plant and the moth, if 
not strictly sustaining the relation of parasite and host, live 
together for a long period, and it approaches much closer to 
mutualism as found between vegetable organisms than phe- 
nomena like entomophily where animals and plants are 
mutually beneficial, without any approach to symbiosis. In 
the vegetable kingdom, mutualism is a relation of mutual 
benefit between organisms living together as parasite and host.’ 
The most conspicuous and earliest observed instance of 
mutualism in the vegetable kingdom is the relation of the 
Lichen fungi to their gonidia or algal hosts. The relation of 
the lichen thallus to its contained gonidia was, at one time, 
the subject of no little ridicule, not only because its discovery 
overturned many established ideas, but because it really did 
seem at variance with common sense. A parasite of far larger 
size than its host, controlling the growth of its host—not grow- 
ing within or upon the host, and following its growth at a dis- 
tance, but growing outside of the host, spreading in all direc- 
tions of its own motion, and being followed by the slower 
growth of the host—such a parasite was indeed a novel phe- 
nomenon. We cannot blame the lichenologists of the old 
school for their facetious remarks about the horse parasitic 
upon the bot and the symbiotic relations of Jonah and the 
whale. 
If all lichens were the large, robust parasites that the com- 
moner lichens are, we should have reason to hesitate long 
before accepting so remarkable a phenomenon as established. 
` ?The case of the bacteria in the « pitchers” of Nepenthes and other carnivorous 
plants seems, according to the investigations of Tischutkin, to been exception. See 
AMERICAN NATURALIST, May 1893. 
