The Causes of Individual Differences 23 



for place and life amongst a multitude of forms which 

 spring in for an opportunity to better their conditions. 

 In a few years more, the tender low herbs have gone. 

 The briers and underbrush have usurped the land. As 

 time goes on, one species after another perishes, and when 

 the place is again reforested, two or three species hold un- 

 disputed sway over the land. The poplars that followed 

 the pines have long since perished and pines again dominate 

 the forest. Or, if the area were turned to pasture a few 

 years after the woods were removed, the herbs and bushes 

 die with the browsing, and in time the June-grass covers 

 the whole landscape with the mantle of conquest. So 

 plants may be said to be always ready to fill new places 

 in the polity of nature by adapting themselves to the new 

 circumstances as they grow into them. The appearance 

 of any one marked variation, therefore, is indication that 

 the plant may have found a new condition, that pressure is 

 somewhat lifted, and that the whole plastic organization 

 may soon respond to. the new environment. It is ap- 

 parent, then, how the simplest and rudest cultivation has 

 been able, through the centuries, so profoundly to modify 

 our domestic plants that we are often unable to recognize 

 the forms from which they have sprung. 



Food supply of different branches. We must not forget 

 to notice, at this point, that the food supply differs amongst 

 the various branches of the same plant. Some branches, 

 by reason of position with reference to the main trunk or 

 with reference to air and sunlight, or, because of a better 

 start in the beginning as a result of some incidental 

 advantage, gain the mastery over others and crowd 

 them out. We have already seen that no two branches 



