PHYSIOLOGICAL CYCLE IN THE INFUSION. 203 



other phenomenon has occurred. The infusoria, originally few 

 in number, finding the conditions favorable, have multiplied 

 enormously, and after a day or two may be seen darting in and 

 out among the bacteria, especially near the surface, and feeding 

 upon them. Among the infusoria, however, are some which 

 feed upon their fellows, so that we soon have the herbivorous 

 infusoria pursued by carnivorous forms, the whole scene illus- 

 trating in one field of the microscope that struggle for existence 

 which is one of the fundamental facts of biology. 



Obviously, this chain of life is no stronger than its weakest 

 part. The hay is the source of the food-supply for all these 

 forms, and this supply must eventually become exhausted. 

 When this happens, the bacteria cease to multiply, the herbivo- 

 rous infusoria which depend upon them perish or pass into a rest- 

 ing stage, the carnivorous infusoria likewise starve, and all the 

 biological phenomena must either come to an end or change 

 their character. 



Up to this point the action is purely destructive. But sooner 

 or later microscopic green plants may appear on the scene, 

 Protococcus, it may be, or its allies, and a constructive action 

 begin, the waste products of the animals and of the bacteria be- 

 ing rebuilt by the green plants into complex organic matter. By 

 this time, also, the dissolved organic matter will have been 

 largely extracted from the liquid, the bacteria for the most 

 part devoured by the infusoria, and the latter may more or less 

 completely have given way to larger forms to rhizopods, roti- 

 fers, small worms, and the like. The putrefying infusion has 

 run its course, and the ordinary balance of nature has been 

 restored. 



Thenceforward an approximate equilibrium is maintained. 

 The green plants build complex organic matter and store up 

 the energy of light. The animals feed upon the plants, or 

 upon one another, break down the complex matter, and dissi- 

 pate energy. The ever-present bacteria break down all the 

 refuse, extract soluble organic matter from the water, decom- 

 pose the dead bodies of the animals or plants, and in the end, 

 it may be, themselves fall victims to devouring infusoria. The 

 physiological cycle is complete. 



