Introduction 17 



perish after they have served their purpose and are 

 replaced each season by entirely new formations. 

 While this development of temporary organs may 

 occur in animals, i.e., the moulting of special plumes 

 in the breeding season of birds, the seasonal de- 

 velopment of the horns of deer, the summer and 

 winter coats of many animals, etc., these organs 

 are not essential ones, and the structures in which 

 they develop are permanent, as for instance the 

 feather and the hair-papillse. 



Plants Less Specialized than Animals. There 

 is, in short, far less difference between the higher 

 and lower forms among plants than is the case 

 among animals. The relative simplicity of even 

 the highest plants perhaps accounts for their greater 

 plasticity. Their immobile condition makes it nec- 

 essary for them to be able to endure the changes of 

 temperature, light, moisture to which they may be 

 subjected, as they have no power to move away in 

 search of more favorable conditions after they once 

 become fixed. Many plants show an extraordinary 

 adaptability for growing under extremely diverse 

 conditions, and may be so changed as to be scarcely 

 recognized as the same species. Thus a tree which 

 grows to a large size in a sheltered valley, may sur- 

 vive near the timber line on a lofty mountain, as 

 a prostrate shrub, rising but a few inches from the 

 ground ; or a weed, growing tall and rank in a damp 

 and shady fence corner, may shrink to a tiny herb 

 with minute leaves and thin wiry stems, when grow- 



