40 MAN, — THE ANIMAL 



Upon this basis, thirty generations of men would 

 be produced in a thousand years. This study upon 

 Paramecium, then, includes more generations by 

 far than the whole of human history. 



It would seem from such a study as if some of 

 these animals could go on indefinitely and that they 

 are in a scientific sense immortal. But other 

 studies covering a wide range of unicellular life 

 show in many instances that these animals actually 

 grow old. Physical changes occur in the proto- 

 plasm which are similar to the conditions found 

 in old cells in the aged higher animals. Death is 

 a part of the life of protozoa as in the higher 

 animals and seems to have many of the same 

 features. There comes a time when these animals 

 grow sluggish in their swimming habits, when they 

 fail to produce offspring and when they cannot be 

 artificially rejuvenated by a change of food or 

 some stimulant; when this stage is reached, the 

 term old age is used to describe it. 



Another aspect of the life of an ameba is one 

 that requires the use of terms usually confined to 

 animals with a definite nervous system. When 

 the ameba creeps about, its streaming protoplasm 

 comes in contact with a wide variety of particles 

 of matter. Some it selects as food; others it 

 passes by in an apparently indifferent fashion; 

 while from another it recoils. Several terms are 

 necessary to describe any one of these reactions. 



