CHAPTER III. 



PROOF OF THE ACTION OF FORCES OF THE ENVIRON- 

 MENT UPON ORGANISMS. — EFFECT OF CHANGES 

 OF ENVIRONMENT. 



The action of the forces of the environment 

 upon the organism can not be accurately measured 

 with the present means at our command, but that 

 such forces do act, and profoundly affect the or- 

 ganism, may be sufficiently proven. We do not 

 understand the laws of this action well enough to 

 predict in all cases what the effects will be ; but 

 when we see a change in the forces of the envi- 

 ronment followed by a change in the forces dis- 

 played by the organism, then we must believe 

 that the former change is the cause of the latter 

 change, for every change must have its cause. 

 We are concerned here only with those changes 

 appearing in the organism, which we call growth 

 and development : we can be satisfied when we 

 can trace the cause of these changes either to 



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