72 
spective hosts, the first important fact indicated is that they are em- 
phatically specific as to the conditions required for their development. 
If this be so, the great differences in the life constitution and food of 
the three species of larvae under consideration would at once render 
mutual intercommunication of their respective diseases impossible. 
The theory held by some that a parasitic germ is readily transmissible 
from one species to another with power to produce disease, must be 
dismissed. Experience has shown that producing disease by artificial 
means in one species furnishes no guarantee that the same germ can 
in like maimer be used to produce disease in a nearly- related species 
and certainly not for those of distant relations. Actual experiment 
may prove it to be possible, which should therefore be done before any 
assertions are justifiable. 
The behavior of the germs in question, under the artificial culture 
conditions recorded in the experiments, indicates that they are faculta- 
tive rather than true parasites. This means that the germs can and do 
under certain conditions, develop as parasitic organisms, but under 
unfavorable conditions can undergo their development in other than 
living matter and thus tend toward saprophytism. Accordingly they 
may gradually adapt themselves to being more saprophytic or more par- 
asitic, whichever the prevailing environment may favor. This is quite 
certainly the nature of these organisms in relation to species of insects 
other than the one which for convenience may be called the natural 
host. Therefore the apparently negative results shown in the experi- 
ments are negative only as concerns the utility of the germ when used 
in the facultative condition in accordance with the usual method of 
procedure. The germs being facultative in their nature, cultivations 
on artificial culture-media begin at once to weaken their power to pro- 
duce disease. When a facultative organism, therefore, is used in the 
usual manner to produce artificial infection, failure is rat*her to be ex- 
pected, and it is manifestly erroneous to consider the results as having 
any direct bearing upon the practicability of parasitic organisms as 
remedial agents. The only interpretation which should be given the 
results recorded in the preceding experiments is that to the insect in 
question (Heliothis armiger) the germs cultivated and experimented 
with, bear only a facultative relation. This fact suggests the abandon- 
ment, as a primary method, of the generally accepted one for experi- 
menting with germs in the attainment of practical economic results. 
This consists in the simple isolation of an organism as a pure culture, 
feeding it to a given insect, and passing final judgment according to 
the results which follow. It further suggests that before the question 
of artificial infection can be satisfactorily solved, the germ used, whether 
really parasitic or only a facultative parasite, must first be studied in all 
its relations to environments which allow the organism to produce dis- 
ease. This done, the next step will be to determine how best to con- 
trol those conditions by artificial means, either in relation to the host 
