500 TEXTBOOK OF BOTANY 



climated, or adapted. From your own experience with lawns, you know 

 that new kinds of plants may come in through seed dispersal, some plants 

 present may die during a hot dry summer, and following the proper 

 application of water and certain fertilizers bluegrass may grow so luxuri- 

 antly that many other plants become overshaded and die. 



Experimental studies of the so-called acclimation of crop plants that 

 have been introduced from one country into another show that some of 

 the species and varieties in the mixed crop simply grew better than 

 others in the new countrv. These varieties produced more seed than 

 those that did not grow so well. Hence as the farmer harvested the seeds 

 each year he obtained a greater and greater percentage of seeds of those 

 plants that were already well adapted to the conditions in the new 

 country. 



The above example refers to experimental studies of a mixed popula- 

 tion of close pollinated plants such as wheat, oats, and barley. If, how- 

 ever, the mixed crop consists of hybrids, or of open pollinated plants 

 that are cross-fertile, various combinations and segregations of factors 

 would occur each year. Here again man, by selecting seeds, would be an 

 additional factor in the final outcome. 



The influence of man is exemplified by our great array of domesti- 

 cated plants, most of which would become extinct within a few years if 

 he were suddenly to disappear from the face of the earth. Exclusive of 

 modern methods of controlling plant breeding, he has been a factor in 

 the evolution of domesticated plants for many thousands of years ( 1 ) by 

 placing the plants close together in his fields and gardens, thus facilitat- 

 ing cross-fertilization, ( 2 ) by introducing plants from other parts of the 

 world that would interbreed, ( 3 ) by cultivating in a way that eliminated 

 ecological influences of one plant upon another ("competition"), (4) by 

 preserving the plant during seasons of the year in which it might other- 

 wise perish, and (5) by selecting certain varieties and eliminating 

 others. 



In the wild state all plants grow in mixed populations. In what ways 

 do these mixed populations, when not interfered with by man, differ 

 from those of crop plants under the conditions of cultivation? In the 

 first place, all their interrelations are dependent solely upon the "laws" 

 of nature. No conscious selecting, preserving, protecting, and distributing 

 takes place. Instead, random (chance) survival, random natural elimi- 

 nation (destruction) by physical factors and by other organisms, and 

 random dispersal checked by geographic barriers are paramount. 



Ever since Darwin adopted the terms "natural selection" and "sur- 



