CHAPTEK XXIX. 
BIOLOGICAL EVOLUTION. 
From the foregoing chapters it will appear that 
there are two lines of forces in action upon every in¬ 
dividual. One is the unchangeable force which holds 
on to ancestral traits; the other is a variable influence 
which is constantly operating to modify them. The 
one reproduces in the offspring the same forms of 
bone, the same flber of muscle, the same digestive, 
circulatory, assimilative, and nervous systems, and the 
same tenor of brain and disposition, as belonged to 
the parent; the other slowly, but none the less cer¬ 
tainly, affects each of these so as to cause signifl- 
cant variations. The one is the force of heredity; 
the other includes, as has been stated, changes in 
climate, soil, and food — indeed, everything that 
touches life from without. All of these may be 
embraced under the term environment, used in a 
somewhat technical sense. Heredity can not origi¬ 
nate life; environment can not act upon life until it 
exists. 
The word evolution signifles the act of unfolding. 
It presupposes something to unfold, and that some¬ 
thing is life. 
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