182 THE ANATOMY OF SCIENCE 



future. In the meantime we must deal with 

 cases which are somewhat hypothetical. 



Let us imagine that a certain solution is 

 capable of producing a given organic sub- 

 stance, but that it will not produce it unless one 

 molecule of this substance is already there, after 

 which more and more of these molecules form 

 at the expense of the nutrient solution. We are 

 assuming that all of these new molecules are the 

 same in composition and structure as those first 

 introduced. Now we may assume further that 

 an isomer of this substance, that is, a substance 

 of the same atoms in a different arrangement, 

 would also reproduce itself in the same solution. 

 Thus by starting with one or the other of the 

 two isomers we could propagate it by inoculat- 

 ing new quantities of the nutrient solution with 

 portions of that which had already been inocu- 

 lated. Or, if both were growing in the same solu- 

 tion, we could form cultures and by careful 

 manipulation separate the one type from the 

 other and afterwards breed them true. 



Now the molecules of such isomers occasion- 

 ally go over into one another, perhaps some 

 collision with a neighboring molecule knocks off 

 an atom or group which falls back into a new 

 position. If then we started with one of these 



