PUFF-BALLS AND TOADSTOOLS. 165 



its host. Death is the certain result. Our 

 house-flies come to grief in this way, and the 

 pity is that more of them did not meet with a 

 like fate. We may often see them on neglected 

 window panes, dead or dying in the centre of 

 a whitish cloud , this cloud is made of mildew 

 threads. After filling the body of the fly they 

 grow and flourish outside a purely fungus 

 growth from a mildew spore lodged in the in- 

 sect. Caterpillars in various parts of the world 

 are carried off in this way. In Australia one 

 may see living caterpillars crawling with a stalk 

 of fungus, more than an inch in height, grow- 

 ing from the head, and sure to kill. Of this 

 fungus group, by some lines of relationship, are 

 the microbes and bacilli that are the causes of 

 diphtheria, consumption, fevers, cholera, and 

 most other diseases of men and beasts. We lit- 

 erally have to contend with powers of the air, 

 an invisible host of living cells that enter our 

 lungs at every breath, flourish in our blood, 

 colonize in our muscles and bones and teeth, 

 and there multiply by growing and breaking 

 in pieces. Without mouths they eat, without 

 stomachs they digest, without organs of gener- 

 ation they multiply by myriads in a day. If it 

 were not that some feed on other kinds we 

 would fare worse than we do. To have lived 



