CHAPTER XII. 



DEGENERATION AND LAWS OF VARIATION. 



The growth of every organism is the result of a 

 certain combination of forces. If any change, there- 

 fore, takes place in this combination of forces, then 

 a change, as the result of their action, must follow. 

 We have seen that a change in the daily illumination 

 of plants can change their periodic sap-flow. Also 

 we have seen that various coloured lights influence 

 differently the development of tadpoles, and that a 

 special diet changes the proportion of the sexes. 

 Exactly how particular forces cause particular ef- 

 fects, we do not understand ; but that they do. act, 

 we are compelled to believe, not only by the general 

 principles of physical science, but by the special 

 experiments upon organisms. In the course of gen- 

 erations, the change of forces produces a change in 

 the hereditary impulse of the organism, and thus 

 a new specific character appears. The hereditary 

 impulse, which is the result of the long previous 



history of the organism, is not immediately changed 



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