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less resistant through removal from their proper homes. Without the 
assistance of commerce the pests of one region would always be lack 
ing, to a greater or less extent, in another; commerce, however, dis- 
tributes them freely. In supplies of grain or other food it has carried 
everywhere the insects which injure stored grain, until nearly all of 
these insects have become practically cosmopolitan. In the more 
recently developed commerce in nursery stock, and the still more 
recently developed long-distance trade in fruit, brought about by rapid 
transit and cold storage, we have carried, and are carrying, potential 
destruction in almost every carload or ship cargo. To such an extent 
has this distribution of injurious insects been brought about, that it is 
difficult in many cases to ascertain the original home of many species. 
Horticulture is perhaps the greatest sufferer from this commercial 
distribution of insect enemies of plants, and the injurious insects most 
readily distributed are the scale insects, since these creatures remain 
attached to the plant throughout their entire life-round. In the 
United States we have, in round numbers, more than one hundred 
species of scale insects, and of these, probably forty have been intro- 
duced from other countries. These forty, moreover, include nearly all 
the species of great economic importance. It seems to be a rule that 
introduced species in this country become far more injurious than in 
their native home, and far more injurious than the species which 
already exist here. It is unnecessary to discuss at length any of the 
reasons for this state of affairs, and in fact, they are not well under- 
stood. In many cases it is due, partly at least, to the fact that the 
imported species did not bring their parasites with them, while in others 
we can only attribute it to the fact that through long association our 
native crops have become more or less immune to the attacks of native 
species, but are less resistant to new enemies. 
A few familiar instances may be mentioned. The oyster- shell bark- 
louse of the apple is a European species introduced into this country 
toward the close of the last century. It speedily became more destruc- 
tive than the native scurfy bark-louse, and during the first half century 
of its existence upon American soil was the principal insect enemy of 
the apple crop; of late it has become less important. The red scale of 
the orange in Florida is an introduction from the West Indies or South 
America; the red scale of the orange in California is an introduction 
from the Pacific islands. The fluted scale or cottony cushion scale 
of the Pacific Coast was originally an importation from Australia. The 
common flat scale and the hemispherical scale of our northern green- 
houses and our southern orchards are European species. The San 
Jose, or pernicious scale, which for twenty years has been seriously 
damaging the orchards of the far west, and which, during the last ir\v 
years, has made a most destructive onslaught on many of our eastern 
orchards, is also probably an Australian species. 
But the scales are by no means the only insects injurious to horti- 
