INTRODUCTION 35 
observations upon the effect of parasites iii the produc- 
tion of neoplasms, incited by Fibiger's discovery of 
nematodes in the rat's stomach cancer, but, with the pos- 
sible exception of a papillomatous growth in the stomach 
of opossums from the action (?) of physaloptera, we have 
been unable to establish such an etiological relationship. 
A decision of the importance of parasites in any given 
case is not without its difficulty, and we are inclined to 
reserve judgment pending further analysis unless the 
effect of the invaders is unequivocal. Leiper (4) 
does not seem to credit animal parasites with a 
great effect on the mortality after a specimen has 
been in the collection six months since all the 
intestinal varieties he studied came from animals dying 
in that period. On the other hand the forms which 
invaded the internal organs and tissues were, in his 
series, from specimens resident several years in the 
garden. He seems to think the conditions of life at the 
garden favor the expulsion of intestinal worms. To what 
extent some intestinal worms may be commensal remains 
as uncertain as the value of certain bacteria in the gut 
tract. In man considerable importance has been ascribed 
to certain fermentative and putrefactive germs in the 
maintenance of a reaction unfavorable to strict pathogens 
and some observers have looked at them as possessing a 
digestive power. In the digestive tract of the animals 
eating large quantities of carbohydrate as cellulose, 
nature provides for its use by rumination and by 
supplying a large hind gut, by which means secondary 
mastication and bacterial decomposition of the cellulose 
capsule insures its full use. Possibly a similar usefulness 
may be finally ascribed to some animal microbes or even 
larger metozoa. 
The role of vegetable parasites in the causation of dis- 
ease among wild animals seems as undoubted as it is in 
the human being and the pathologic results are usually as 
(4) Proceedings, Zoological Society, London. 1911, p. 620. 
