112 DOCTRINE OF EVOLUTION 



inorganic influences which first come to mind when the 

 term is used. For example, the environment of a lion 

 includes other lions which are either members of its 

 own family, or else, if they live in the same region, 

 they are its more or less active rivals and competitors. 

 In the next place, other kinds of animals exist whose 

 lives are intimately related to the lion's life, such as 

 the antelopes or zebras that are preyed upon, and the 

 human hunter to whom the lion itself may fall a victim. 

 In addition, there are the contrasted influences of inor- 

 ganic nature which demand certain adjustments of the 

 lion's activities. Light and darkness, heat and cold, and 

 other factors have their direct and larger or smaller 

 effects upon the life of a lion, although these effects 

 are less obvious in this instance than in the case of 

 lower organisms. 



The reality of variations due to the inorganic elements 

 of the environment is everywhere evident. Those 

 who have spent much time in the sun are aware that 

 sunburn may result as a product of a factor of this 

 class. The amount of sunlight falling upon a forest 

 will filter through the tree-tops so as to cause some 

 of the plants beneath to grow better than others, thus 

 bringing about variations among individuals that may 

 have sprung from the myriad seeds of a single parent 

 plant. In times of prolonged drought, plants cannot 

 grow at the rate which is usual and normal for their 

 species, and so many variations in the way of inhibited 

 development may arise. 



Then there are the variations of a second class, more 

 complex in nature than the direct effects of environ- 

 ment, namely, the functional results of use and disuse. 



