CHAPTER III. 



PROTOPLASMIC AGE OF PROTOZOA. 



Upon watching one of these simple organisms through the micro- 

 scope there is a certain fascination in the idea that this minute bit of 

 naked pi-otoplasm has been continuously living since life appeared 

 upon the earth. As a matter of fact, the same sensations might be 

 experienced upon gazing at any of our fellow-beings, or, indeed, at any 

 other living thing; but somehow we do not think of the latter in this 

 way; we associate with them the ideas of age, of senile degeneration 

 and natural death, concepts which do not seem to be associated with 

 the free-living cell. It would appear, furthermore, that the ameba 

 protoplasm which we see under the microscope, and which has lived 

 continuously for all of these ages, might continue to live for an indefi- 

 nite time in the future. It would seem that this perfectly balanced 

 cell, with its powers of growth and reproduction, would be self-suffi- 

 cient, containing within itself the potential of an endless existence. 

 Such, however, is not the case, protozoa, like metazoa, may die of old 

 age. 



In every higher animal we recognize certain more or less definite 

 periods of physiological activity, and according to these we roughly 

 divide the span of life into three stages, which are in no way sharply 

 outlined. These we call the stages of youth, adolescence, and old age. 

 Youth, characterized by a high degree of vitality, is the period of rapid 

 cell multiplication and growth; organs are formed and perfected, 

 functions are unimpaired and active and the body is a perfect living 

 thing. The second period is characterized by functional and sexual 

 maturity; the multiplication of tissue cells is less rapid; the organs 

 strengthen and their functions are more perfectly correlated; growth 

 comes to an end. In the perfected animal it is the period for per- 

 petuation of the race, and in conformity with this great function 

 sexual differentiation is fully established. The third period, old age, 

 brings a marked change, the potential of vitality wanes, cells atrophy, 

 and functions weaken; degenerations of all kinds appear; and cumu- 

 lative weakness ends in natural death. 



These three periods are characteristic of all of the higher many- 

 celled animals, the last period being rarely seen in nature, because in 

 the wild animals a violent death follows the earlv fiuictional weakening 

 and inability to fight off enemies. Do we find the same sequence of 



