388 
by the facts. The introduction of species 
from Europe into North America is of 
every-day occurrence and their establish- 
ment is far from rare. The carriage of 
American species to Europe is an equally 
frequent matter, but their establishment is 
much less frequent. 
I have studied in this connection—my 
profession being that of an economic ento- 
mologist—principally the species which are 
prominent as injurious to horticulture or 
agriculture or in other ways inimical to 
man. Listing the insects of prime economic 
importance in the United States, the species 
each of which almost annually causes a loss 
of hundreds of thousands of dollars, we find 
that they number seventy-three. Of these 
thirty are native, while thirty-seven species 
have been introduced, six species being of 
doubtful origin. Of the thirty-seven intro- 
duced species, thirty have come to us from 
Europe, all, with one exception, as acci- 
dental importations. 
Of the prominent European injurious in- 
sects, on the other hand, but three are said 
to have come from America; the grape-vine 
Phylloxera (Phylloxera vastatria), the woolly 
root-louse of the apple or ‘ American blight’ 
(Schizoneura lanigera), and the Mediter- 
ranean flour moth (Ephestia kuhmiella). Of 
these but one is certainly American—the 
Phylloxera. The origin of the Schizoneura 
is somewhat doubtful, while the Mediter- 
ranean flour moth is not American, but 
probably came to us from Europe, although 
originally it is probably Oriental. 
As with these insects of prime economic 
importance, so it is with other less noted 
species. There have been rather frequent 
establishments of European species in 
America, but practically none of American 
species in Hurope. The reason for this 
curious condition of affairs is difficult to 
find. The general trend of accidental im- 
portations seems to have been westward, 
and it is doubtless a fact that certain of our 
SCIENCE. 
[N. 8S. Von. VI. No. 141. 
now cosmopolitan forms were originally 
Asiatic and have traveled westward, 
through Europe, to and across America, 
and thence to Hawaii, New Zealand and 
Australia. The existence of such a law is 
borne out in the study of plants as well. 
The statement just made regarding insects 
of prime economic importance is almost ex- 
actly paralleled with the plants classed as 
weeds. It has thus been shown that, out of 
two hundred American weeds, one hundred 
and three are introduced, of which ninety- 
six are from Palearctic regions, sixty-eight 
being native to Europe, while it seems that 
less than half a dozen American species 
have become troublesome in Europe. A 
number of American species, however, have 
been carried to Australia and flourish there 
with vigor. 
This general trend from east to west has 
always been in the direction of the newer 
civilization—from the older civilization to 
the newer. That thisin itself is significant 
cannot be doubted, and in the case of the 
insect and plant enemies of agriculture the 
facts surrounding this condition are al- 
most in themselves sufficient to account for 
this directive movement. J have shown in 
another paper that the denser population 
of the older countries and the resulting 
vastly smaller holdings in farms, the 
necessarily greatly diversified crops, the 
frequent rotations of crops, together with 
the clean and close cultivation necessitated 
by the small size of the holdings, and the 
cheaper and more abundant labor, will all 
operate as a barrier against the establish- 
ment of injurious species, while the reversed 
conditions in a newer country at once 
liberate an introduced species from the re- 
pressive conditions which affected it in its 
original home and encouraged its establish- 
ment, multiplication and spread. But 
there are deeper causes than this at work. 
It has been suggested that the flora and 
fauna of America, the older continent, have 
