808 THE POPULAR SCIENCE MONTHLY. 



be assumed that each of the long list of diseases known as infec- 

 tious is caused by its own specific virus, and that no other ma- 

 terial or combination of agencies can produce it. This fact is 

 universally recognized. Furthermore, it has been demonstrated 

 in a large proportion of these diseases that the essential principle 

 of infection in the virus is the living germ called the pathogenic 

 microbe. 



The literally vital relation of microbic to human life can be 

 observed in the following general statement : The pathogenic mi- 

 crobes cause four fifths of all diseases of the human family ; they 

 destroy more lives than war, famine, fire, murders, shipwreck, 

 and all other casualties ; and they actually abbreviate the aver- 

 age natural term of human life by three fourths, and constantly 

 depress the health average of the world's population far below its 

 natural standard. 



They are an insidious but powerful and relentless enemy to 

 human kind, holding sway over a large part of the most beauti- 

 ful and fertile portions of the earth, excluding man at the peril of 

 his life; while, as if with malicious discrimination, ferocious 

 animals and venomous reptiles find there their congenial home, 

 and vegetation reaches its acme of luxuriance. Like some dia- 

 bolical spirit, in the form of the epidemic, it leaves its native 

 habitat, and with insatiate malignity, sometimes with slow but 

 irresistible progress, and again by rapid flight, passes all barriers 

 of mountain, sea, and distance in its pursuit of man, its only 

 known object, and whose destruction is its only visible effect. 

 This is shown in Asiatic cholera, the plague, yellow fever, and the 

 lesser scourges. The strife for possession in some coveted regions 

 has been progressing for ages. Man may advance his outposts 

 under the favoring light of sunshine, but must retreat, or fortify 

 himself against the dangerous shades of night, until, by slow de- 

 grees, advantages are gained over the invisible enemy. 



The Italian Pontine marshes, the jungles of India, the banks 

 and shores of the tropics, our own Southern lowlands and fertile, 

 new prairies are the strongholds of Bacillus malarial. 



"The pestilence that walketh in darkness, and the sickness 

 that wasteth at noonday," are known to be the work of the armies 

 of the microbes. 



Some of the means and methods of the micrologist, in his re- 

 searches, must be mentioned. His outfit is extensive and novel. 

 It includes the best known microscopes and a well-constructed 

 incubator with heater and thermometer, numerous test-glasses, 

 beakers, filters, acids, alkalies, deep-colored dyes, and a good sup- 

 ply of prepared cotton. 



In studying the life history of his microbes he will require a 

 well-supplied commissariat. He must be a professional caterer 



