94 THE POPULAR SCIENCE MONTHLY. 



Already the practical man may be told, in reply, that surgery is 

 entirely reformed by our knowledge of the minuter fungi ; that, by 

 avoiding the access of bacteria to wounds, we avoid a large destruction 

 of human life. Already we see our way to avoiding some deadly dis- 

 eases caused by these same bacteria now that we know them to be the 

 active cause of such disease. Already silk is cheaper in consequence 

 of our knowledge of the bacteria of the silk-worm disease ; already 

 better beer is brewed and better yeast supplied to the baker in conse- 

 quence of Pasteur's discovery of the bacterian diseases of the yeast- 

 plant ; already vinegar-making, cheese-making, butter-making, wine- 

 making, and other such manufacturing trades are on the way to bene- 

 fit by like knowledge. Potato-disease and coffee-disease have been 

 traced to their causes and means suggested by biologists for dealing 

 with the parasitic plants causing those diseases, whereby not thousands 

 but millions of pounds sterling a year may be saved to the commu- 

 nity. 



Insect-pests which have depopulated whole provinces, such pests 

 as the phylloxera and the Colorado beetle, are about to receive a check 

 at the hands of the same class of scientific students. The application 

 of knowledge of natural facts is in this case a very remarkable one ; 

 for it is actually proposed to make use of our recently acquired knowl- 

 edge of diseases due to bacteria not that we may arrest such diseases, 

 but that we may promote them. Insect-pests are to be destroyed by 

 poisoning them not with acrid mineral poisons which damage plants 

 as well as the insects, but by encouraging the spread of the disease- 

 producing bacteria which are known to be fatal to such insects. Pro- 

 fessor Hagen, of Cambridge, Massachusetts, has called attention to the 

 old practice of destroying greenhouse pes"ts by the application of yeast. 

 He conceives that this method may be applied to other insect-pests, 

 such as phylloxera, Colorado beetle, cotton-worm, etc. He imagines 

 that the yeast-fungus enters the body of the insect on which it is 

 sprinkled, and there produces a growth which is fatal to the insect's life. 

 It is a well-known fact that insects are very subject to fungoid dis- 

 eases, and it is also ascertained that the application of yeast to the 

 plants frequented by such insects favors their acquisition of such dis- 

 ease. Professor Elias Metschnikoff, the celebrated embryologist has, 

 however, made some investigations on this subject, and given an ex- 

 planation of the possible value of yeast application (" Zool. Anzeiger," 

 ~No. 47), different and more satisfactory than that which Professor 

 Hagen appears to adopt. 



The general result of the most accurate investigations of the beer- 

 yeast fungus (Saccharomyces cerevisice) is entirely opposed to the notion 

 that it can enter an insect's body and produce a disease. Beer-yeast 

 is beer-yeast and appears always (or within experimental limits) to re- 

 main so. On the other hand, De Bary has made known the life-history 

 of some simple fungi which destroy insects, and from Pasteur, Cohn, 



