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 diseases 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 consequence of Pasteur's discovery of the bacterian diseases of the yeast-plant; already vinegar-making, cheese-making, butter-making, winemaking, and other such manufacturing trades are on the way to benefit 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 community.
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 knowledge 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. Professor Hagen, of Cambridge, Massachusetts, has called attention to the old practice of destroying greenhouse pests 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 diseases, and it is also ascertained that the application of yeast to the plants frequented by such insects favors their acquisition of such disease. Professor Elias Metschnikoff, the celebrated embryologist has, however, made some investigations on this subject, and given an explanation 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 cerevisiæ) 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 remain so. On the other hand, De Bary has made known the life-history of some simple fungi which destroy insects, and from Pasteur, Cohn,
