Plants in Relation to Man 479 



kinds of plants are usually found, suited to the conditions, 

 and maintaining their kinds in the unceasing struggle. When 

 climbing vines begin growth and continue to obtain sufficient 

 nourishment, they may not only overgrow trees, but finally 

 deprive them of light and cause their death by starvation. 

 The survivors are necessarily few in number, but they have the 

 qualities that insure their continuance and the propagation of 

 their kinds. Thus it appears that nature carries on a thinning- 

 out process as unceasingly and relentlessly as the expert gardener 

 who destroys weeds in protecting and encouraging his selected 

 plants. The -process as conducted in nature is called natural 

 selection, and as carried on by man is called artificial selection. 



In thinning out young vegetables, the smallest are usually 

 removed and the largest, healthiest, and best located are left 

 to mature. Natural selection, however, does not always result 

 in the survival of the largest or best, according to man's view. 

 In forests the largest trees may survive in the struggle for light, 

 forcing the small and unsuccessful to perish in the shade, but 

 in drought the small, unshapely plant with long roots and few 

 or small leaves may survive ; or it may be that one which 

 best endures frost. On an individual tree the branches and 

 twigs that grow most rapidly and vigorously crowd out of the 

 race those that seem to be dormant. It is the fitness of indi- 

 viduals in a given set of conditions that insures survival and 

 reproduction of their kinds. 



Conditions of natural selection : prodigality of nature. Obser- 

 vation of the struggle for existence among plants reveals the 

 conditions upon which natural selection depends. The first 

 of the conditions is the prodigality of nature, evidenced in the 

 production of more individuals and species than can find ade- 

 quate room and food for full growth and reproduction. This 

 makes inevitable a competitive struggle for existence. Any 

 plant, wild or cultivated, produces hundreds of seeds, only 

 one of which will perhaps take root, grow, and mature. If a 

 dandelion produced fifty seeds, each of which in turn produced 



