ILLUSTRATED AMONG PROTOZOA. 115 



conditions become more complex. We must bear in 

 mind that all changes in the constitution of the or- 

 ganism are the result of the action of forces which 

 are too numerous and intricate for us to follow. The 

 illustrations here given, however, will show the nature 

 of their action. Thus we see that, during the course 

 of life of an individual, both the constant and peri- 

 odical forces acting upon it produce a gradually 

 changing series of effects, as the state of the or- 

 ganism is gradually changed by its continued reac- 

 tions. These unchanged forces act as a gradually 

 changing series of- stimuli, and the life of the organ- 

 ism is thus a gradually changing series of reactions. 

 The reactions follow each other in a certain regular 

 order, as the organism changes in size and complex- 

 ity, and they are the same in each succeeding gener- 

 ation. When the same protoplasm, or continuity of 

 protoplasm, has repeated this series several thousand 

 times in as many generations, we can easily under- 

 stand how all the successive reactions in the series 

 become so firmly associated and co-ordinated that 

 they all tend to follow the first stimulus, even though 

 many, or even half, of the succeeding original stimuli 

 remain in abeyance. 



Suppose all the forces of the environment to remain 

 the same for a great length of time. As each genera- 

 tion differs from the preceding generations in the 



