636 FUNDAMENTALS OF FRUIT PRODUCTION 



try develops and remains chiefly in those sections or districts where fac- 

 tors governing production, harvesting, distribution and marketing are 

 most favorable. In other words, the present geographic distribution of 

 the different fruit industries represents the result of a struggle for 

 existence, a real natural selection. 



Summary.^ — The most important environmental factor determining 

 the geographic range of cultivated fruits is temperature, though rainfall 

 and humidity act as important limiting factors within the wider limits set 

 by temperature. The boundary lines of fruit zones follow rather closely 

 those of the life zones established by the biologist. Apparently, mini- 

 mum winter temperatures are most important in setting the northern 

 limits (in the Northern Hemisphere) to the geographic range of species 

 and varieties and mean summer temperature during the hottest 6 weeks 

 in setting their southern bounds. The limiting effects of natural rainfall 

 are often alleviated by the use of irrigation water or by other cultural 

 practices and also by the selection of drought resistant varieties. Sun- 

 shine, wind and the presence of certain parasites are often important 

 factors in determining the range of particular varieties. There is no 

 very close relation between the length of time a species or variety has been 

 in cultivation or between the natural range of related forms and its 

 range in cultivation. Artificial factors, such as nearness to large centers 

 of population, transportation and storage facilities, and temporary 

 market demands, often are of considerable importance in determining the 

 approximate range of a variety or of a fruit and in determining its 

 elative importance within different portions of its range. 



