152 DEVELOPMENT AND HEREDITY. 



normal life of a land animal. The amount of differ- 

 ence is immaterial ; we may suppose it greater or less. 

 In the production of this difference there are two 

 kinds of influences at work. First, a negative influ- 

 ence, from the withdrawal of the heretofore normal 

 stimuli of growth, and second, a positive influence, 

 in the application of an entirely new system of stim- 

 uli ; and, in either case, we must remember that 

 these stimuli form a system, i.e. by their sameness 

 and constancy they tend always to produce a certain 

 definite impression on the organism, — a certain defi- 

 nite direction of growth. A system of stimuli calls 

 forth a corresponding system of reactions, which 

 comprise the nutrition, growth, development, and 

 other exhibitions of energy by the animal. Through 

 frequent repetitions in previous generations, this sys- 

 tem of reactions has become welded together in all 

 its parts by the power of association ; therefore, 

 when some of the stimuli are withdrawn, the in- 

 herited impulse is not immediately affected. So 

 that, after a number of generations had led an 

 aquatic life, if the ordinary stimuli of exercise on 

 land were restored, the young would develop after 

 the fashion of its land ancestors, as has generally 

 been observed to be the case in similar circum- 

 stances. While we do not understand, with any 

 exactitude, the laws of association and "forgetting," 



