THE ORIGIN OF LIFE. 17 



only serve to show us that there is something else 

 beyond, which is not comprehended. Our first task 

 is then to find out exactly what is the question now 

 at issue ; for it is very different from what it used to 

 be. We shall find that much is now universally con- 

 ceded which was at one time strenuously disputed. 

 There are three essential properties possessed by 

 living things which must be included in any at- 

 tempted explanation of life : a. Their constant 

 activity, b. Their power of growth, c. Their power 

 of reproducing themselves. These being the essen- 

 tial properties of life, their satisfactory explanation 

 will bring us far toward the understanding of life 

 itself. 



Relations of Organic Activities to Physical Energy. 



First we may say that the activities of organisms 

 are no longer looked upon as manifestations of a 

 distinct " vital force " unrelated to other forces. It 

 will hardly be denied by any one to-day that all of 

 the energy exhibited by organisms in their various 

 activities is a part of the store of energy of the uni- 

 verse, and that all of the forces exhibited by animals 

 are correlated with physical forces in general. It 

 has been conclusively proved that every motion 

 made by animals, every bit of heat a/ising in them, 

 is simply a portion of the energy which this world 

 has received from the sun. The process of its trans- 

 formation is as follows : 



Plants, by virtue of the possession of a body called 

 chlorophyll (i.e., their green coloring matter), have the 

 power of using the energy of sunlight. By means of 



