186 TILLERS OF THE GKOUND CHAP. 



vapours come from, whereas Pliny thought they 

 came from the Dog-star, but these are the chief 

 differences between the two. 



If, instead of taking a plant disease, we had 

 taken such a disease as malaria we should have 

 found almost the same thing ; that is, that for many 

 hundreds of years practically no progress had been 

 made in determining the exact cause of the disease. 

 It is a minute animal parasite which causes 

 malaria, and it is a plant parasite which causes 

 mildew in wheat, but in neither case could progress 

 be made until other sciences had advanced. Before 

 minute parasites can be studied we must have 

 powerful microscopes, and it took a very long time 

 to find out how these could be made. 



Once again, a very great deal of the progress 

 which has been made in recent years in finding out 

 the causes of disease, has been due to the fact that 

 men of science have a great number of dyes at their 

 disposal to colour the minute animals and plants 

 which cause disease. Even with very good micro- 

 scopes it would not be possible to find out the 

 details of the structure of these parasites, if it were 

 not possible to " stain " them, as it is called, with 

 certain dyes. Now most of these dyes are what 

 are called " aniline dyes," which are obtained from 

 coal-tar, and it was only in the middle of last 

 century that these dyes were discovered. 



