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- 
tieular 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 loeal 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 
