The Causes of Individual Differences 25 



the remaining parts receive an amount of food in excess 

 of the habitual allowance. 



5. By divergence of character in associated plants. 

 It is well known that a field planted so thickly to corn 

 that it cannot grow more with profit, may still grow 

 pumpkins between. The pumpkins and the corn are so 

 unlike in form that they complement each other, the one 

 filling the place which the other is not fitted to occupy. 

 We have already seen that a copse ever so full of bushes 

 may still grow vines. A meadow full of timothy may still 

 grow clover in the bottom, and land covered with apple 

 trees still grows weeds beneath. " The more di versed the 

 descendants from one species become in structure, con- 

 stitution, and habits," writes Darwin, "by so much will 

 they be better enabled to seize on many and widely diver- 

 sified places in the polity of nature, and so be enabled to 

 increase in numbers." 



Variation in climate. The fact that any distinct 

 climatic region usually has plants that are very closely 

 related to those of other climatic regions in the same 

 zone, points strongly to the probable profound modifica- 

 tion of plants by climate. And, furthermore, we should 

 expect that if the food environment modifies plants, the 

 climatic environment must have the same power. More- 

 over, there is abundant historical and experimental proof 

 that climate is capable of greatly modifying the vegetable 

 kingdom. There are those who contradict any great effect 

 of climate in the variation of plants, and acclimatization 

 has been even stoutly denied. These persons make the 

 mistake of asking that a visible modification take place 

 at once upon the transfer of a plant from one climate to 



