28 LECTURES ON BIOLOGY 



movement present in all fluids. We may obtain a similar effect 

 by adding very fine particles of soot to a drop of water, when 

 these tiny grains of soot will be observed to dance and vibrate 

 within the field of vision as if they were alive. 



That, on the other hand, rest, or the absence of perceptible 

 motion, may make it very difficult to distinguish between life and 

 death, has been demonstrated by the following experiment made 

 by Verworn. If we examine under the microscope a drop of 

 the sediment in a bottle of white Berlin beer we shall see in the 

 fluid numerous minute globules of a pale colour, being suspended 

 singly or in rows of twos and threes. However long we may 

 continue our examination, these globules always remain in perfect 

 rest, without exhibiting the least trace of movement. Similar 

 formations, exactly alike in appearance, may be observed in milk. 

 But whilst the latter are nothing but inanimate particles of fat, 

 the globules in the beer are organized unicellular vegetable 

 growths, the familiar yeast-cells which cause fermentation of the 

 beer. Many other analogous .cases might be mentioned. Even 

 the trained microscopist frequently finds it difficult to decide 

 whether certain minute bacilli, spirilla, or cocci occurring in 

 preparations are bacteria or decomposition-products of the sur- 

 rounding tissues. Mere appearance is not sufficient for deciding 

 whether an organism is an animal or a plant, or for determining 

 whether an object is animate or inanimate. 



Physiology has therefore always striven to determine the 

 difference between dead matter and an organism. But the results 

 at which the different investigators arrived were widely diver- 

 gent, according to the view-point of the investigator. While 

 one side denies that there is anything in common between the 

 two and will at the most admit that organisms are built up 

 with the same chemical elements that are found in inorganic 

 nature, the other side recognizes none of these distinctions as 

 valid. The extreme standpoint among modern physiologists is 

 taken by Verworn, who even refuses to accept metabolism as an 

 exclusive characteristic of organic substance. 



As long as each organism lives it performs work, and work 

 requires force. As a steam engine can only continue to work 

 as long as it is fed with fuel, so each organism must consume 

 food, from the combustion of which its energy is obtained, 



