THE GROWTH OF BIOLOGICAL IDEAS 167 



spontaneously generated but arise only from pre-existing 

 micro-organisms naturally had a profound influence on the 

 development of bacteriology; for it was at last obvious that 

 exclusion of the germ meant exclusion of the disease. Early 

 in the seventies a German investigator, C. J. Eberth, per- 

 formed the experiment that linked Davaine's with Pasteur's. 

 He filtered the deadly blood of animals suffering from an- 

 thrax and showed that the filtrate ^vas innocuous. There was 

 nothing in the filtered blood that could multiply and cause 

 disease, and the germs could not be generated spontaneously. 

 Bacteriology now made rapid strides, thanks largely to ad- 

 vances in technique. Robert Koch introduced valuable 

 methods for making bacteria readily visible under the micro- 

 scope by staining them, and he also discovered how to grow 

 them outside the body on jelly in glass vessels, a technique 

 that is still in use today. 



Microscopists now looked confidently for the germs of the 

 most diverse diseases; but their confidence was misplaced. 

 It was soon discovered that some diseases could be artificially 

 transmitted from one animal to another, as are diseases caused 

 by germs, despite the fact that no sign of any micro-organism 

 could be detected under the microscope. Pasteur considered 

 that such diseases must be caused by micro-organisms too 

 small for the microscope to resolve. Diseases of this kind 

 were found to occur also in plants. And now, in the last 

 decade of the century, Eberth's filtration experiment was 

 found not to be universally valid. It was shown that the juice 

 of a tobacco plant infected with mosaic disease would cause 

 the same disease in previously healthy plants even if the juice 

 were filtered. Something had been discovered that could 

 only be observed through its effects on organisms; this some- 

 thing had the power of self-multiplication but, unlike ordi- 

 nary germs, could pass through a filter. This was the starting 

 point of our knowledge of the filter-passing viruses, which 

 are the cause of so many diseases of man, such as smallpox, 

 chicken pox, measles, German measles, influenza, and com- 

 mon colds. 



