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 



