FUNDAMENTAL PRINCIPLES 41 



Expression of this cellular condition is greatly modified 

 by circumstances, which are never precisely the same 

 for any two individuals (C/. p. 48) . 



31. 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 V and VI. 



32. Adjustment to Envirotunent. — By the term 

 environment is meant all the circumstances that surround 

 a cell, tissue, or organism at any given time, or throughout 

 its existence. The environment of tissues and organs 

 includes surrounding tissues and organs, and the environ- 

 ment 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 necessitates an adjustment on the part 

 of the plant in order to maintain this harmony. Adjust- 

 ments 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 its roots deeper, if hght is entirely cut off no 

 chlorophyll will form. A leaf, or the prothallus of ferns, is 

 bilaterally symmetrical partly because the environment is 

 uniform on all sides; the same organs have dorso-ventral 

 differentiation largely because the environment is unlike 

 above and below. The motility of sperms is an adjustment 

 to water in the environment. Thus, variations in the 



