CHAPTER IV 



Genetic Variatiokin Unicellular Organisms, with Special 

 Relation to the Inheritance of Environmental Effects 



and Reactions 



THE remaining chapters will be devoted to some of the 

 types of observed genetic variation in unicellular organ- 

 isms; particularly certain types that do not fit well any of the 

 categories of genetic variation thus far considered/ Genetic 

 variations in unicellular organisms show relations to environ- 

 mental conditions and to their reactions to these conditions, 

 that are of interest for any general understanding of the 

 nature of genetic variation. 



Unicellular organisms differ from higher organisms in 

 several obvious ways that are of great importance for genetic 

 variation. First, genetic variation, in order to be effective, is 

 not here restricted to a group of germ cells hidden away in 

 the interior of a large body, as is the case in the higher organ- 

 isms; for here the whole body is but one cell. Second, the 

 entire organism is very small; in some of the unicellular 

 organisms it is indeed so minute as to be beyond the reach of 

 the ordinary microscope. In consequence, all of its materials 

 are in close relation with the physical and chemical conditions 

 of the environment, and exposed to their action. And third, 

 parent and offspring are not separated by a long series of cell 

 generations. In the higher organisms, the offspring begin as 

 single cells, and a long and complex process of development 

 occurs, involving thousands or millions of cell divisions, be- 

 fore the adult condition is reached. All this is lacking in 

 unicellular organisms. Reproduction here occurs by a single 



