The Evolution of Medical Science. 147 



in for blame as factors in the causation of disease. Sanitary 

 science, therefore, began its development along this line, 

 and fortunately, although the actuating ideas proved at a 

 later date to be in the main erroneous, they led on to actual 

 progress. The laws of ventilation were thus discovered, 

 and the benefits of pure air came to be appreciated. Not, 

 however, until bacteriology evolved into a distinct depart- 

 ment of science, did we have clear ideas as to how pure 

 air and pure water operated in bringing us health. Then 

 the mistakes of sanitarians, during what we may for con- 

 venience call the Chemical Era, began to be righted. We 

 discovered that chemically and mechanically filthy air, and 

 chemically and mechanically filthy water, may be relatively 

 almost harmless, while air and water that, to all our tests, 

 would seem absolutely pure, might deal death and destruc- 

 tion to us. The old rule that pure air and pure water are 

 necessities for the maintenance of proper health still holds 

 good ; but we now see that there are apparent exceptions 

 that even seem to reverse this truth. It all depends upon 

 the nature of the germs present. These germs are minute, 

 microscopic plants (not animals), which, like all other forms 

 of vegetable life, require more or less decomposing materials 

 to grow upon.* Their numbers, kinds and natures are 

 legion. Some will grow on acid, but not on alkaline or 

 neutral soil. Others will grow on alkaline soil only. Still 

 others on neutral. Some grow at very high temperatures, 

 and perish at low ones. Others are in their element at low 

 temperatures, and die at high ones. All sorts of poisonous 

 solutions have special kinds, capable of maintaining healthy 

 growth within them. 



Anything capable of killing bacteria of all kinds, must 

 necessarily be of such a nature as will kill everything 

 having life. The adaptations of all bacteria cover and go 

 beyond every adaptation of man, so that, in all directions, 

 man must be killed easier than bacteria are. This, re- 

 member, is only true of them as a total. Take any one 

 kind, and man's adaptations transcend its own, so that it 

 is much easier to kill it than man. Of the innumerable 

 kinds known, only a comparatively small number are able 

 to grow in or upon the human body as a soil. These have 

 their choice of parts. Some like to grow on the skin, 

 while others prefer mucous membranes, and still others pre- 



Cradle's Germ Theory of Disease, p. 6. 



