142 DEVELOPMENT AND HEREDITY. 



a certain round of activities, — birth, development, 

 growth, and reproduction. This round takes place 

 over and over again, the result of the same round 

 of stimuH acting on living matter in each genera- 

 tion, with only slight variations, which, as already 

 explained, are generally added only at the end of 

 the round. Now as this round, which we call the 

 life of the individual from inception to reproduction, 

 repeats itself, there is acquired not only facility 

 of response to stimuli, but those responsive actions 

 become associated together by the fundamental 

 property of nervous association, in such a way that 

 the chain of associated activities becomes almost 

 inseparable. On the same principle that a thought 

 in the mind calls up an associated thought, or one 

 tone of music calls up another, or one action in 

 an oft-repeated series of- actions calls up the next 

 subsequent action or actions, so the initial stimulus 

 being given to an incipient organism, its responsive 

 activity each time tends to produce by association 

 the next activity in the oft-repeated series, and 

 so on through the successive steps of growth and 

 development. By association the tendency to 

 develop in a given direction is already present ; but 

 in point of fact the same succession of stimuli 

 which has acted upon previous generations is also 

 continually present to compel the same routine 



