118 DEVELOPMENT AND HEREDITY. 



Other causes arise a constant succession of changes 

 in the environment of organisms. Thus the combina- 

 tion of forces acting upon all organisms change 

 more or less slowly. Thus the relation between 

 the external forces of environment and the internal 

 forces of the organism is changed ; and some new 

 adjustment of the structure and condition of the 

 organism must be the result. As a species of 

 organisms develops into more highly organised 

 and more complex beings, the later generations 

 will come under the influence of new stimuli occa- 

 sioned by their own wider range of activity. New 

 stimuli of this kind, too, can be effective in pro- 

 ducing new developments only after the individuals 

 of each generation have attained, in their onto- 

 genetic development, to that degree of growth 

 which brings them into contact with the new 

 stimuli, — just as a child cannot be developed by 

 practising the physical and mental pursuits which 

 are necessary for the development of a young 

 man. New stimuli, thus brought to bear on an 

 organism, are effective only toward the end of the 

 already acquired course of development. 



The course of development may be summed up 

 as follows : the primitive mass of protoplasm with 

 which we started acquires nervous co-ordinations 

 which influence its activity and growth ; as it divides 



