144 DEVELOPMENT AND HEREDITY. 



causes of growth and development. The forces 

 of the environment may remain the same, but the 

 instant they act upon an organism they produce a 

 change in the organism ; this change puts the organ- 

 ism in a new relation to the forces of the envi- 

 ronment, which again alters the action of the 

 environment upon the organism ; i.e. makes it a 

 new stimulus, and a new change in the organism 

 ensues, and so on, to an apparently infinite extent. 

 I have used the word " stimulus " as indicating the 

 action of a force on an organism at any one instant. 

 The stimulus is dependent on the relation of the 

 forces of the organism to the forces of the environ- 

 ment ; while the latter may remain unchanged, 

 the organism changes, and the relation between 

 the two being changed, the stimulus is changed, 

 i.e. the force effects the organism differently. 



The inherited impulse, which is produced by the 

 association of an often-repeated series of activities, 

 must, from its nature, be strongest in the initial 

 stages of the life of an individual, for those stages 

 have been oftenest repeated. The impulse will be 

 weakest for those characteristics which were the 

 last to be developed in the species, for these have 

 been repeated the least number of times, or gener- 

 ations. We see thus the aptness of Darwin's method 

 of statement, that "characteristics become fixed by 



