CHAPTER V 



Genetic Variation in Unicellular Organisms, Continued. 

 What Is the Method of Operation of the Genetic System 

 in Bringing about Genetic Variations under the Long- 

 Continued Action of the Environmental Conditions? 



THE preceding chapter set forth that in unicellular organ- 

 isms, changed environmental conditions eventually 

 cause corresponding changes in the characteristics; and these 

 changed characteristics may be inherited in later generations. 



The occurrence of the "inheritance of acquired characters" 

 in any group of organisms is of great interest, and is w^orthy 

 of careful examination as to its method and significance. Such 

 an examination w^e undertake in the present chapter. We 

 ask: How does the organic system operate in bringing about 

 genetic variations under the long-continued action of the 

 environmental conditions? How does it happen that a 

 changed environment, although it finally alters the inherited 

 characteristics, does not give an inherited effect until it has 

 acted for many generations ? How does it happen that resto- 

 ration to the original conditions finally causes a return to the 

 earlier characteristics, but not till after many generations ? 



Something is known as to the nature of the organic system 

 that operates in this way; something is known as to its 

 method of operation. Examination of these matters will com- 

 pel us to consider some details in the structure and physiology 

 of the organisms ; it will illustrate to what lengths nature goes 

 in making difficult a correct interpretation of her activities. 



New inherited characteristics acquired under the action of 

 the environment are essentially modifications of the organic 



