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 
predominatess 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 obvious 
than the results of adaptation, the latter was almost 
