4 GENERAL PHYSIOLOGY 



less consciously by the same criterion as the primitive races, who 

 from the fact of motion, considered as living the dancing flame of 

 a fire or a moving wave. In fact, of all vital phenomena, motion 

 is the one that gives most strongly the impression of living. 



It may be said that only primitive races and children are misled 

 by the criterion of motion, and that the civilised and adult man, who 

 is versed in a knowledge of life, is capable of deciding easily in any 

 given case between the living and the lifeless. But this is not 

 always true. For example, are dried grains living or lifeless ? Is 

 a lentil that has lain unchanged in a chest for years living ? 

 Scientific men themselves are not agreed upon this point. The 

 lentil, when dry, does not show phenomena of life, but, if placed in 

 moist earth, it can at any moment be induced to do so. It then 

 sprouts and grows into a plant. 



The decision between the living and the lifeless becomes, how- 

 ever, much more difficult with objects that are not commonly seen 

 in daily life, e.g., certain microscopic things. Long observation 

 and very detailed investigation are frequently required in order to> 

 determine whether certain bodies that are found in a liquid by 

 microscopic examination are living or not. If a drop of the dregs 

 be taken from a bottle of weissbeer and examined with the micro- 

 scope, it will be found that the liquid contains innumerable small 

 pale globules, often clinging together in groups of two or three,, 

 completely at rest so long as they are observed, and showing no 

 trace of movement or other change. Very similar small globules, 

 may be observed with a microscope in a drop of milk. The two 

 kinds of globules can be distinguished from one another by strong 

 magnifying powers only. No trace of vital phenomena can be 

 found in either by the most patient and continued microscopic 

 examination, yet the two objects are as widely different as a living 

 organism and a lifeless substance ; for the globules from the beer 

 are the so-called yeast-cells (Saccharomyces cerevisicz), the active 

 agent in the fermentation of the beer and fully developed, unicellular, 

 living organisms, while the globules from the milk are lifeless; 

 droplets of fat, which, by their abundant presence and their reflec- 

 tion of light from all sides, give to the milk its white colour. As 

 a counterpart to these two objects, we may consider a third. In 

 the body-cavity of the frog on either side of the spinal column 

 between the transverse processes of the vertebrae there lie small y 

 yellowish-white masses. If a bit of the contents of one of these 

 be removed with a knife and placed with a drop of water upon a 

 slide, and the whole be covered with a cover-glass, there may be 

 seen with strong powers of the microscope a mass of minute 

 granules and short rods of different sizes, which are trembling and 

 dancing in constant motion, the smaller particles very actively, the 

 larger ones more slowly. Every untrained person, brought before 

 these three preparations and asked which of the three objects- 



