Influence of Environment 167 



times unfavorably to the growth or development of a par- 

 ticular part or function. There is therefore maintained by 

 the species the observed tendency to cluster around its typical 

 or modal form. It is only rarely that conditons continue for 

 a long period altogether favorable or altogether unfavorable. 

 Both by direct observation and by experiment we can 



Fig. 40. Influence of Environment Upon Plants 



Two plants of Gaura parviflora, a and b, grown respectively in shade and in sunshine. 

 Two forms of Solidago, c, the species oreophila and d, the alpine form of the same, 

 decumbens, dwarfed through low temperature and excessive loss of water on account of 

 the low atmospheric pressure at high altitudes. After Clement. 



find that prolonged exposure to extreme conditions does re- 

 sult in a considerable modification of the development. Ex- 

 perimentally a plant may be split into two parts and one 

 grown at a high elevation and another in the valley: the end 

 results are strikingly different individuals (Fig. 40). One 

 half of a plant may be submerged in water while the other 

 is allowed to root into the soil and grow into the air. Again 

 the ensuing individuals are strikingly different in their struc- 

 ture, appearance and behavior, although strictly speaking 



