The Causes of Individual Differences 27 



it tends to vary to meet the uncongenial environments. 

 In the high North, many plants are so variable that the 

 marks used to identify the species in other latitudes are 

 often lost. 



8. There may be a profound variation or modification 

 in constitution and habit by which plants become ac- 

 climatized, or enabled to endure a climate at first injurious 

 to them. This may occur by a variation in the constitu- l 

 tion of the descendants, which enables them directly to 

 endure more untoward conditions. It generally comes 

 about, however, through a change in habit, by which 

 plants, when transferred towards the poles, shorten their 

 season of growth or even become annuals. Plants become 

 more sensitive to spring temperatures in cold climates, 

 so that they start relatively much earlier in the season 

 that is, at a lower sum-temperature than in warm 

 climates. Any one who has passed the springtime in both 

 the North and South must have noticed how much more 

 suddenly the vegetation comes forward in the North; 

 and it is surprising how the spring-sown crops accelerate 

 their growth in the North over those in the South. 



Man's control over climate as a means of making plants 

 to vary. The characters that result from a change of 

 climatic environment are peculiarly within the control 

 of the agriculturist, for a leading factor in his business 

 is the transfer of plants far and wide over the earth. So 

 it has come that the staple varieties of the important 

 grains and fruits are unlike in Europe and America and 

 in all great geographical areas, although all the various 

 forms may have sprung from one ancestor within historic 

 times. A new country is stocked with varieties from 



