II 



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 bf 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- 

 ly seen ; or bhghts 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 



' See Nature for March II, 1880. 



