214 AGRICULTURE. 



the higher plants those species covering a wide 

 ranee show ereater variation in their individual 

 members than species more restricted in their 

 distribution — conditions which might be ex- 

 pected; the more diverse the environment, the 

 more variable the individual. Thus, the luxu- 

 riantly growing plant at the base of a moun- 

 tain varies greatly from its dwarfed brother at 

 the summit (Figs. 53, 54). On the other hand, 

 the more variable the plant the more easily it 

 can adapt itself to varying conditions; hence, 

 the more widely it is distributed. 



I. Causes of J'ariation. — Variation is not the 

 result of chance, yet the detailed differences in 

 varieties of the same species can only be sug- 

 gested. 



In every individual two factors are manifest: 

 the nature of the organism, and the nature of 

 the external conditions. Nevertheless, the same 

 conditions do not always produce the same re- 

 sults, for similar varieties may be "produced^ 

 from the same species under external conditions 

 of life as different as can well be conceived, and, 

 on the other hand, dissimilar varieties may be 

 produced under apparently the same condi- 

 tions." Variation, then, may be due, for the 

 most part, to the innate tendency of the organ- 

 ism to vary, the causes of which are not fully 

 understood. However, some of the causes of 



* Darwin's Origiit of Species, p. 127. 



