FUNDAMENTAL PRINCIPLES 189 
into a mature organism, resembling the one from which 
it came, but under other combinations of circumstances 
the external appearance the expression may resemble 
that of the parent only a very little, or not at all. In- 
heritance may therefore be defined as the recurrence in suc- 
cessive generations, of a similar cellular constitution. Ex- 
pression of this cellular condition is greatly modified by 
circumstances, which are never precisely the same for 
any two individuals. 
171. Variation. The preceding sentence explains, in 
part, why it is that no two individuals are ever precisely 
alike precisely similar circumstances surrounding de- 
veloping organisms never occur twice ; that is, the environ- 
ment varies. Besides this, internal changes may take 
place in the reproductive cells. For either one or both of 
these reasons, constant variation is the rule for living 
things. This subject will be considered more at length 
in Chapters XXXII and XXXIII. 
172. Adjustment to Environment. By the term envir- 
onment is meant all the circumstances that surround a 
cell, tissue, organ, or organism at any given time, or 
throughout its existence. The environment of tissues 
and organs includes surrounding tissues and organs, 
and the environment of cells includes the neighboring 
tissues and cells. The most essential thing in the 
life of every plant or animal is to keep in harmony with 
its environment. Every change of environment neces- 
sitates an adjustment on the part of the plant in order to 
maintain this harmony. Adjustments are most easily 
made when the plant is young and plastic, and especially 
while it is developing to maturity. If the amount of 
water in the soil is diminished the young plant will send 
