Inheritance of Variations 79 



longer spines ; others shorter ; others more numerous spines ; 

 others fewer (Figure 24). Different masses change in dif- 

 ferent ways ; the number of kinds of diversity that we get is 

 large; apparently indefinitely large. The protoplasm cer- 

 tainly gradually becomes diversified as it continues to exist 

 and increase. 



A number of important questions at once arise. What 

 part of the protoplasm is it that thus changes? Is it the 

 cytoplasm, or the nucleus, or is it both? And how does 

 the change occur? Through irregularities in the division 

 of certain substances or parts? Or through chemical or 

 physical changes produced by the environment, by changes 

 in temperature, chemical composition of the water, by food, 

 or the like? And why do we find these changes to occur in 

 Difflugia when we could not find them in Paramecium ; when 

 in almost all the other organisms studied in this way such 

 changes have not been discovered? 



All these questions are bound up together. We shall 

 perhaps deal best with them by taking up the last question 

 first. Why do we find such a difference in this respect be- 

 tween Difflugia and other organisms, for example, Para- 

 mecium? 



As was set forth earlier, Difflugia was selected for study 

 precisely because it was much more favorable for such work 

 than Paramecium or most other organisms investigated. In 

 Difflugia there are many well defined distinctive characters, 

 which are not modified by growth, nor changed by the con- 

 ditions under which the animals live. In Paramecium and 

 most other forms studied, on the other hand, the characters 

 are continually changing through growth and environmental 

 action. Such changes are well known not to be inherited. 

 This makes a tremendous difference as to accomplishing 

 anything by selecting any particular character. Thus in 



