THEORIES OF DEVELOPMENT 



Origin of Life. Many scientists to-day advocate that life 

 is the action of chemical and physical forces in connection with 

 a peculiar substance called protoplasm. But no one has yet 

 been able to explain all the phenomena of life by means of these 

 forces. Until this is done, we must believe that life from life 

 is the universal rule, or that no living organism originates except 

 from some pre-existing living form. 



Protoplasm is the physical basis of all life, both plant and 

 animal, and without this complex substance life cannot exist. 

 It is a protein which is known only as a product of living sub- 

 stances. 1 It is chemically and physically unstable, but it is 

 impossible to obtain a satisfactory chemical analysis since the 

 dead material differs from the living protoplasm both in its 

 power and structure. 



There is little direct proof of the character of life in the 

 Archaean Era, but it must have been marine. Since plants 

 usually feed upon inorganic matter and animals cannot manu- 

 facture organic compounds from inorganic ones, and as plant 

 life is thus a necessity for the existence of animal life, it is 

 thought that plant life may have been first. However, the 

 view that plant and animal forms originated side by side and 

 have developed along diverging lines is common. 



Since the simplest form of life to-day consists of a single cell, 

 it is believed that primitive life began as a single cell. 



The Cell Theory.- Cells were first described by Hooke, an 

 Englishman, in 1665, and were so named from the resemblance 

 of the compartments in the structure of a piece of cork to the 

 cells in a monastery. Schleiden, in 1838, showed that plants 

 were composed of cells, and in 1839 Schwann discovered the 

 same general fact concerning the bodies of animals, thus re- 

 vealing the common plan of organization of plants and animals. 



The cell theory involves: (1) that all organisms are made up 



1 McFarland's " Biology." 

 382 



