168 THE LOWEST ORGANISMS AXD THE CELL 



Protoplasm lias the power of reproduction. Reproduction ac- 

 companies growth and depends upon cell division. Of course 

 the surface area of a cell cannot increase in the same ratio as 

 its bulk. The surface is the region of the cell through which 

 some of the most important life processes of assimilation and 

 respiration take place, and a certain amount of cell surface is 

 necessary for a given bulk of protoplasm. Therefore the cell 

 divides when, after a period of growth, the bulk of protoplasm 

 becomes proportionally too great for the amount of surface area. 

 The sum total of the surfaces of the daughter cells is materially 

 increased by their division, while the combined bulk of their 

 protoplasm remains the same as before. 



A living being is like a machine in that it requires fuel to 

 generate its energy or power of doing work, but the organism 

 has the peculiar ability of making its own repairs, of increas- 

 ing in size, and of detaching from itself portions which can in 

 their turn attain the structure and efficiency of the parent. The 

 process of life is continuous, although the material of protoplasm 

 is constantly changing, that is, substances are constantly 

 going into the organism and substances are going out. It may 

 be compared to a whirlpool in a river: the form and action of 

 the whirlpool is constant, although the water which enters and 

 leaves remains for only a short time in circular movement. 



Protoplasm always comes from preexisting protoplasm. This 

 means that protoplasm, so far as we know, never springs into 

 existence from inorganic material. It is never formed de novo. 

 There have been naturalists and philosophers who believed that 

 life might arise spontaneously under favorable conditions in 

 suitable nutrient solutions. They cited such illustrations as 

 the swarming microscopic life which appears in extracts or infu- 

 sions of animal and vegetable matter as examples of sponta- 

 neous generation. These theories were overthrown chiefly by 

 the work of Pasteur and Tyndall, who showed that life never 

 appears in these extracts and infusions provided proper care 

 is taken to kill all organisms that may be in them, together 



