VIII. 



Heredity — Reversion — Variability — Adaptation — Results of Use and 

 Disuse of Organs — Differentiation leading to Perfection. 



The two properties of organic being which determine 

 and regulate the relation of the offspring to the pro- 

 genitors, and which not only assign to individuals their 

 position in the surrounding world, but also help them 

 to attain it, are transmission or heredity, and adaptation. 



Heredity is the conservative, adaptation, the pro- 

 gressive principle. Yet all heredity is not directed to 

 immutability, and many cases of adaptation involve 

 morphological and physiological retrogression. For the 

 elucidation of the inherited peculiarities of organisms, we 

 reconstruct their pedigree; by the characters acquired 

 by adaptation, we test the pliability of organisms in the 

 lapse of time, and trace the ramifications of the pedigree. 

 Groups of organisms, in which the conservative principle 

 predominates, certainly evince their powers of endurance 

 in the struggle for existence, but they make no advance 

 in physiological value, and are outstripped by the more 

 progressive groups which yield to obstacles and profit by 

 them, a course of which human life also affords so many 

 examples. 



As the phenomena of heredity are usually more obvi- 

 ous than the results of adaptation, the latter was almost 



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