112 Chapters in Alodern Botany chap. 



harmless. What it requires hard boihng to accomplish, 

 the sunlight does with rapid ease. For although the 

 spores or young bacteria which are carried in the air are 

 peculiarly hardy, able to nurse their dormant virulence for 

 months or even years, ready when they find a suitable 

 resting-place to multiply with appalling swiftness, they 

 cannot withstand the power of the sunlight. What is the 

 light of life to other plants, is to the bacteria death. Per- 

 haps it makes them live for a short time so rapidly that 

 their scanty speck of living substance is worn out and 

 consumed beyond possibility of repair. Quite literally 

 then "the pestilence walketh in darkness." Hence then 

 we may interpret much of the history of disease, and see that 

 we have here no mere mysterious visitations, to be crouched 

 before with submissive dread ; but a definite and intelligible 

 part of the order of nature not only to be analysed and 

 grasped by science, but which we may partly modify by art 

 and partly by adapting our other conditions. And thus it is 

 that we find so many dead germs in the air ; thus we regard 

 all dark places as evil, whether they be the lidless coffins 

 of the slums or the dungeon-like sunk flats of west-end 

 houses ; thus we begin to appreciate the hygienic importance 

 of sunlight — the most universal, the most economical, the 

 most potent antagonist of our subtlest and deadliest foes. 



But the part which bacteria play in the economy of 

 nature is often beneficent. For although we may not derive 

 much satisfaction from the fact that bacteria by thinning out 

 the weaker organisms help to keep up the standard of 

 vitality, a little observation leaves the more cheering con- 

 viction that many of them are among the great cleansers of 

 the world. Out of dead plants some form vegetable mould, 

 while others reduce the carcases of animals to simpler and 

 purer substances. Consider the large circle on which 

 bacteria occupy an arc ; green plants feed on air, water, and 



