98 DEVELOPMENT AND HEREDITY. 



each individual, that we must look for the causes of 

 the diversity of form and habit among organisms. 

 The action of these stimuli is cumulative in its 

 effect. After a stimulus has acted upon an organ- 

 ism, and the organism has returned to what is called 

 its " normal condition," we must not suppose that 

 the second normal condition is the same as the 

 condition of the organism before the action of the 

 stimulus ; for the stimulus has caused a molecular 

 change, and this change persists until some other 

 force undoes it or intensifies it. The viscous living- 

 matter retains its impression, and is more impression- 

 able than a solid body, of which Professor Maxwell 

 has said, "the stress at any given instant depends, 

 not only on the strain at that instant, but on the pre- 

 vious history of the body." When a stimulus has 

 acted upon an organism, and called forth a respon- 

 sive change or action, then at the second time 

 that stimulus acts, and the third time, and always 

 afterward, the tendency is to produce the same 

 response. If we were now to take account of all 

 the stimuli acting on an individual during its life- 

 time, we would find that the vast majority of them 

 are constantly repeating themselves. By observing 

 the world around us, we see that in the lives of 

 animals — man included — each day is very much like 

 the day before. The stimuli and their responsive 



