SEPTEMBER 10, 1897. ] 
with others it is the unexpected which has 
happened. As Osten Sacken says, speaking 
of the Diptera: “‘Importation will not oc- 
eur for centuries in cases where it might be 
expected from day to day; and, again, it 
will sometimes take place under circum- 
stances most improbable, and, a priori im- 
possible to foresee.”’ (Proc. Entom. Soc. 
Lond., 1894, p. 489.) 
' Why should the well-known Pieris rape 
have made its appearance in this country 
and spread far and wide, while the equally 
common and injurious Pieris brassice and 
P. napi have never been found here? Why 
should Phytonomus punctatus have flourished 
with us when it is hardly known as a 
clover enemy in Hurope, and when the 
congeneric Phytonomus meles of Europe has 
never been found here? Why should Cole- 
ophora laricella have established itself here, 
and none of the other European Coleoph- 
oras (some of them of much greater dis- 
tribution and hibernating in cases of pro- 
tective coloration and shape, and attached 
to plants) have acclimated themselves 
amongst us? Why should Calliphora vomi- 
toria, Orytoneura stabulans and Stomoays calci- 
trans have been brought over at an early 
date and flourished to excess in America 
and many other countries, while Sarcophaga 
carnaria is unknown in any of them ? 
Mr. Schwarz has phrased it : “ We stand 
here before some great unknown factor, 
viz, the individual character and inmost 
nature of the species which governs the 
introduction or non-introduction of each 
species—a factor which is variable accord- 
ing to each species * * * .” But there is 
is no reason why a mystery need be made 
of this condition. In a word, it is the ca- 
pacity of the individual species to accommo- 
date itself to a more or less novel environ- 
ment. Nowhere in the whole animal 
kingdom do we find the natural environ- 
ment more complicated than with insects. 
Conditions are frequently dependent upon 
SCIENCE. 395 
conditions in an almost endless chain. The 
phenomena of fatal parasitism are of vital 
importance as determining the abundance 
of the species and are curiously complicated. 
I have recently proved the existence of 
several fatal tertiary parasites and the 
probable existence of quaternary parasites 
with Orgyia leucostigma in Washington. 
Upon the condition of this chain of inter- 
dependencies rests the welfare of the 
primary host. If adverse conditions affect 
the quaternary parasite, the primary host 
suffers, for the tertiary parasites increase 
and kill off the secondary parasites, allowing 
an increase of the primary parasites which 
kill off the Orgyia. The famous instance 
of Darwin in which he showed thatin a 
measure cats are responsible for the produc- 
tion of clover seed in England through the 
interrelations of cats, field mice and 
bumble-bees, is paralleled and outdone again 
and again among insects. Further, in no 
group of animals are the characteristics 
termed special protective resemblance and 
special aggressive resemblance, to say 
nothing of protective and aggressive mim- 
icry, so well marked and so important in the 
life of the species as with the insects. It is 
upon the degree of simplicity of its life—the de- 
gree of simplicity of its normal environment as a 
whole—that the capacity of a species for trans- 
portation and acclimatization, even into a parallel 
life zone, depends. 
Nevertheless, I am fully convinced that 
very many more species will stand trans- 
portation from the Palearctic to the 
Nearctic, from the Australian to the Ori- 
ental and the Neotropical regions, than 
would be supposed from a consideration of 
these points and from a knowledge of the 
comparatively few forms which have as yet 
been transported and acclimatized. Aside 
from the forms brought in with their food 
and thus under the most favorable condi- 
tions for establishment, it is only by a 
lucky chance with the average accidental 
