194 THE STUDY OF ANIMAL LIFE CHAP. 



a period of extraordinary senility. This may be avoided 

 by taking great precautions to keep up the food-supply 

 and to remove waste-products. In certain cases the 

 senility of the race of Infusorians can be staved off for 

 a time by using appropriate stimulants, such as beef- 

 tea, and very perfect nutritive media have been dis- 

 covered in the course of experiment. In a stock all 

 descended by asexual multiplication from one ancestor 

 there is no conjugation probably because they are all 

 practically identical. It has been supposed by some 

 that the absence of conjugation is in itself the cause of 

 the decadence of the stock, but it seems more likely that 

 the function of conjugation is to promote variability. 

 In natural conditions this variability may enable a 

 strain to parry successfully some change in the environ- 

 ment. 



The combination of all the vital activities within the 

 compass of a single cell involves a very complex life 

 within the unit, not more complex than the entire life 

 of a many-celled animal, but fuller than that of one of 

 its component cells. While a Protozoon is relatively 

 simple in structure, its life of crowded functions, such 

 as moving, digesting, breathing, is exceedingly complex. 

 The simpler an organism is in structure the more difficult 

 will it be to study its separate functions. Physiological 

 or functional simplicity is in inverse ratio to structural 

 or morphological simplicity. Thus the physiologist 

 makes most progress when he seeks to understand 

 animals with many parts, for there he can find a large 

 number of units, all as it were working at one task. 

 The life of a Protozoon is more manifold and complex 

 than that of any unit from a higher animal, just as the 

 daily life of the savage at once hunter, shepherd, warrior 

 is more varied than ours. 



Every many-celled animal, if it is reproduced in the 

 ordinary way, begins its life as a single cell, as an egg- 

 cell with which a male element has united. Thus it 

 may be said that every Metazoon begins its life at the 

 level of a Protozoon, and this is true even of very large 

 animals, for the whales arise from ova " no larger than 



