68 ORGANISMS OF ONE CELL 



nated on the earth's surface from salts and other inorganic 

 matter at a time when conditions of temperature, atmosphere 

 and other physical characteristics of the globe were very differ- 

 ent from the conditions today. At the present time, while 

 ignorant of the first causes all are agreed that living matter 

 cannot arise spontaneously from non-living matter, and that all 

 plants and all animals come from the germs of their ancestors. 

 All theories to the contrary have been based upon ignorance, 

 and the gradual clearing away of these dark clouds forms an 

 interesting chapter in modern biology. A characteristic of the 

 human mind is to explain what cannot be seen or comprehended, 

 by the most plausible hypotheses based upon what is known. 

 This is why in the lyth 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; these 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. RedPs 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. 

 This theory, therefore, finally died 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 contained in 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 



