VARIATION. 93 



In the improved families of pure-bred animals, the 

 influence of artificial conditions in modifying charac- 

 ters is further shown in the excessive development 

 obtained in special directions. 



The principal causes of animal variation are cli- 

 mate, food, and habit ; the influence of the first two, 

 in many cases, being so intimately connected that it is 

 difficult to determine what is due to each, while all of 

 them may at times act together. Of the many illus- 

 trations of the modifying influence of climate that 

 might be drawn 'from the vegetable kingdom, we 

 shall only present some general statements in regard 

 to two of our leading crops. 



Indian-corn (maize) has a wide geographical range, 

 but in its distribution and development it is influenced 

 in a great degree by climatic conditions. In North 

 America its extreme limits at the North " are defined 

 by the isothermal of 67 for July, and it may go be- 

 yond 65 for the summer ; one month, however, being 

 required at a higher mean than this." ] 



In Northern Europe, including Great Britain, the 

 comparatively low summer temperature prevents the 

 ripening of this valuable cereal, although it is grown 

 in some localities as a forage-crop. 



The time required for ripening the crop in locali- 

 ties where it is grown varies greatly with the climate. 

 In its extreme northern range, where the smaller va- 

 rieties only are grown, but from two to two and a half 

 months are required to bring it to maturity, while at 

 the South a period of from five to six months is neces- 

 sary. 



1 Blodgett's "Climatology of the United States," p. 420. 



