xvi Life and Living Creatures 313 



and increasing the amount of carbonic acid in the atmo- 

 sphere ( 154). Those plants which do not contain chloro- 

 phyll, such as the fungi, moulds, and bacteria, are as powerless 

 as animals to manufacture food from carbonic acid and water. 

 But unlike animals they have the power of manufacturing 

 proteids if they obtain starch or sugar and the various salts 

 amongst their food. Thus fungi all the mushroom kind 

 grow abundantly only in decaying vegetable substance, 

 which supplies plenty of starch. To sum up in a metaphor, 

 the green plant, like a coal-laden steamer, conveys solar 

 energy using up some in the process to the animal, which 

 like a stationary steam-engine converts it into work. 



401. Micro-organisms. Many minute one-celled organ- 

 isms, probably plants allied to the fungi and moulds, known 

 as bacteria, bacilli, microbes, or classed together as micro- 

 organisms, play a very important part in the course of their 

 life-history. One of these, known as the nitrifying ferment ', 

 changes the salts of ammonia derived from the atmosphere, 

 or from decomposing animal matter, into nitric acid in the 

 soil, thereby greatly facilitating the growth of plants ( 311). 

 Another known as yeast, when cultivated in a weak solution 

 of sugar, uses up some of the sugar, and changes the rest into 

 carbonic acid and alcohol, hence it is extensively used in rais- 

 ing bread and in making wine and beer. A different micro- 

 organism changes alcohol in the presence of air into vinegar, 

 and is extensively cultivated for that purpose. The spores, 

 or young undeveloped cells, of many kinds of micro-organisms 

 form a considerable part of the dust in air ( 161), and are 

 present everywhere. When these find a suitable place to 

 grow in for example, the blood or the tissues of a person not 

 in strong health they develop and multiply, producing by 

 their vital processes certain poisons, which give rise to disease. 

 Different species of micro-organisms have been detected as 

 the cause of cholera, consumption, diphtheria, and other 

 maladies. The recognition q this cause of disease has led 

 within the last few years to the greatest modern advances 

 in medical treatment. 



402. Evolution. As the Earth, like other members of 

 the solar system, is the result of a slowly unfolding series of 



