VARIATION IN UNICELLULAR ORGANISMS 109 



through long subjection to a weak solution of them a power 

 to transform the harmful substance into something else that 

 is innocuous; practically to destroy it. It was found that each 

 of the above named organic substances induces a different 

 change in the animals, so that infusoria that have acquired the 

 power to resist and destroy quinine do not resist and destroy 

 methylene blue, and vice versa; and similarly for the other 

 substances. The acquired resistance is specific for each sub- 

 stance. Thus the materials of which the organism is composed 

 — or certain of these materials, at least — are changed in a 

 different way by each of the chemicals to which resistance is 

 acquired. 



But what is most important for our purposes, such acquired 

 resistance is inherited, in the free-living Protozoa as well as 

 in the parasitic ones. The organisms are removed from the 

 chemical that has induced the resistance. They continue to 

 multiply by fission, and their descendants for hundreds of 

 generations continue to possess the resistance acquired by their 

 ancestors. In the course of a great number of generations 

 under conditions lacking the chemical, the increased resis- 

 tance may gradually decrease and finally disappear; to this 

 fact we return later. 



But the positive fact of inheritance for hundreds of gener- 

 ations shows clearly that the organisms, or certain material 

 of the organisms, have become changed under the action of 

 external conditions, and that this modified material con- 

 tinues to assimilate, increase, reproduce, in its modified con- 

 dition. This modified, more resistant substance increases 

 itself a thousandfold in ten generations; many millions of 

 times in a hundred generations. It is clear therefore that the 

 environment has produced a genetic variation; an inherited 

 variation. 



