1921] Riley: Guide-posts to Medical Entomology 161 
biology. Though this influence was an intangible one, it must 
nevertheless be reckoned with. However important this intan- 
gible influence may have been and however fundamental bac- 
teriology has been to the development of medical entomology, 
the outstanding guide-posts of a century and more ago were in 
the field of helminthology, or the study of the grosser parasites 
of man and animals. 
We laugh today at the crudities of the early zoologists who 
taught in the most matter-of-fact Manner, amazing theories of 
heterogenesis, according to which the offspring of a given 
animal may be something utterly different from itself. How- 
ever deeply ingrained they were in the popular mind, theories 
of the development of geese from barnacles, or of the spon- 
taneous generation of frogs from mud, of mice from grain, and 
of bees from dead oxen were discarded by the advanced scientists 
of the 17th century. 
Much more slowly did this changed viewpoint apply to 
the various helminths or intestinal worms. We know that Redi 
with all of his keenness and daring was unwilling to commit 
himself to the view that the endo-parasites of animals might 
not originate spontaneously. It is more surprising to find that 
as late as 1819 the noted parasitologist Bremser presents a 
careful, and, for the period, convincing argument in favor of 
the spontaneous origin of these forms. The noted medical 
treatise by Roche and Sanson, in 1828, declares that the Oppos- 
ing view according to which the helminths are transmitted 
from animal to animal, being introduced into the body through 
the air, or food, or drink, demands such a measure of credulity 
that it is surprising that it has found any defenders. 
But even as Bremser wrote he and scores of indefatigable 
workers were accumulating data which were to completely 
reshape the attitude of the zoological and medical world towards 
these parasites. In the first place, there was a growing rec- 
ognition of the fact that the grosser parasites which, from the 
time of the ancients, had been known to occur in the bodies of 
man and animals, did not constitute a group absolutely apart, 
but were animals subject to the same fundamental laws of 
development as were free-living forms. Thus helminthology, or 
the study of parasitic worms was recognized as having more 
than medical interest, and attracted the attention of leading 
zoologists of the period. 
