20 The Principles of Fruit -growing. 



given soils. But all this forced adaptation to soils 

 is a very special matter, and it only illustrates the 

 more strongly the great importance of giving par- 

 ticular attention to the general subject of the adap- 

 tabilities of species, varieties, and even of strains, 

 to variations in soils. 



The parasite determinant. Inasmuch as many of 

 the organisms which seriously interfere with fruit- 

 growing are more or less restricted in their range, 

 it would seem to follow that the zones of profit- 

 able fruit -culture may be determined more or less 

 by the parasite factor. A moment's reflection will 

 show, however, that the geographical distribution of 

 the parasite is determined primarily by climate and 

 by the distribution of its host -plants; so that, on 

 the one hand, the climatal limit of the cultivation 

 of the fruit may be approximately the climatal dis- 

 tribution of the pest, and, on the other hand, the 

 parasite is local or cosmopolitan according as the 

 fruit is either local or widely grown. 



Many of the common pests are restricted in range 

 because they have not yet reached the full limit of 

 their distribution. An excellent illustration of this 

 fact occurs in the case of the codlin-moth. A 

 generation ago, Michigan was represented to be the 

 Eutopia of the apple -grower because of the absence 

 of this pest, and in our own day similar recommen- 

 dations have been made of Oregon and other far 

 western states. To the naturalist, however, it was 

 evident from the first that the insect was following 

 closely behind the apple frontier, as a storm follows 



