634 Destruction of Obnoxious Insects by Fungoid Growths. [Sept'r, 
Metschnikoff, might in some cases prove destructive to insects to 
which it had been applied, not because of the yeast itself, but 
because of impurities which it might contain, it being an 
established fact that yeast may and often does contain the living 
spores of more than one kind of fungus. 
One of the things which is always taken into account in the 
cultivation of fungi in the laboratory, is the fact that when we 
purposely sow the spores of a given fungus we are never quite 
sure that we are not at the same time unconsciously sowing the 
spores of some other fungus which may be floating in the air. 
While we expect and generally obtain an abundant crop of the 
fungus we may wish to cultivate, we are apt to find here and 
there one or more other forms mingled with those we are trying 
especially to grow. These may be compared to the weeds which 
the gardener is pretty certain to find among the plants he has 
sown in his seed bed. 
The subtle nature of fungoid growths of every grade, and the 
peculiar dependence of fungi upon climatic conditions and other 
circumstances, which are often unknown and wholly beyond con- 
trol, are very important factors in estimating the probabilities of 
success by the methods under consideration. We know that 
epidemics and epizootics of various kinds may be largely destruc- 
tive and fatal one year, and the next, although the germs of 
disease must now be scattered in abundance everywhere, the 
disease is lessened or is wholly gone. Again, rust and smut may 
one season abruptly invade our grain-fields ; the next it is scarce- 
y seen; or blights and mildews may devastate for a few years our 
orchards and vineyards, and then gradually or suddenly disappear. 
Nor are these characteristics confined to microscopic fungi alone. 
During last season certain of the larger species, as Boletus and 
Hydnum, were sought in vain in localities about the University, 
where ordinarily they were abundant. The fall had been un- 
usually dry and in this probably lay the cause of their non-ap- — 
pearance. Perhaps for the same reason, not a house-fly could be 
found that was affected with the Empusa, though they were 
sought for with much care, especially about the Botanical Labora- 
tory where the yeast fungus was being grown. 
Nevertheless, inasmuch as it is possible to suppress injurious 
fungi which destroy our economic insects, as for instance, the — 
1 See Naturé for March 11, 1880. 
