SPONTANEOUS GENERATION 67 



by the most plausible hypotheses based upon what is known. 

 This is why in the iyth century it was generally believed that 

 insects are spontaneously generated in decaying meat, the myth 

 being disproved by the simple experiment of Redi, an Italian 

 naturalist who kept fresh meat under a fine netting upon which 

 he saw flies deposit their eggs which he watched develop into 

 maggots and later into flies. Thus a set of phenomena was 

 removed from the unknown into the known and Redi concluded 

 from his observation that all living things come from pre- 

 existing living things, a conclusion formulated in the well-known 

 dictum credited to Harvey (?) omne vivum ex vivo. Redi's con- 

 clusions however were not to be fully accepted for more than two 

 hundred years for shortly after his experiments were carried 

 out the world of microscopic organisms was discovered by the 

 Dutch naturalist Leeuwenhoek, and ignorance of their origin 

 resulted in a new life for the theory of spontaneous generation, 

 and a life which finally died v out only in our own times with 

 the famous experiments of Pasteur and Tyndall upon the 

 smallest visible forms of living organisms, bacteria and yeasts. 



The Problem of Age and Natural Death. With full and un- 

 hindered processes of metabolism there is no a priori reason why 

 living matter as shown by the single- celled organism apart from 

 tragic or accidental death, should not live indefinitely. So 

 far as we know however all living things experience a weakening 

 of these fundamental biological activities and pass through a 

 longer or shorter period of physiological weakness which we term 

 old age ending in natural death. The length of life varies 

 within wide limits, animals on the whole having shorter lives 

 than plants, some of the giant trees of California for example 

 living for tens of centuries while some of the insects (may- 

 flies) are born and live out their life within a single day, al- 

 though other animals like the tortoise may live for hundreds 

 of years. 



Senescence. In the higher animals or metazoa the cells are 

 differentiated for the performance of different functions and the 

 various activities of metabolism are relegated to different types 

 of specialized cells. These are the links in the chain of vital 



