OF WASHINGTON. 183 
occasions more or less extended lists, which,' however, are mere 
fragments, because they served only to illustrate certain special 
features. It is certayily strange that hitherto a complete list of 
such species has not been published.* In following up the his- 
tory of many of these species it is interesting to see how much 
difficulty has been and is still experienced by entomologists in 
coming to an understanding regarding the identity or non-identity 
of species from widely-separated regions, difficulties that illus- 
trate the powerful influence of difference in habitat upon our 
conception of the term species. Usually the species were first 
described independently under different names, then, either by 
comparison of specimens or descriptions, they were declared 
to be identical ; then there came a period when more or less 
minute differences were pointed out between such species ; 
then a reaction set in in the opposite direction, and this state of 
uncertainty regarding many species will no doubt continue for an 
indefinite time. A complete list of the Coleoptera common to 
both continents can also not be given as long as certain smaller or 
larger portions of our fauna are not more fully studied or, at any 
rate, not yet compared with the Old World's species. Here be- 
long the whole family of Cryptophagidce, the subfamily Aleo- 
charince of the Staphylinidce with an almost endless number of 
species, and some other genera of various families. Incomplete 
as it is, my list comprises 440 species. f It is, however, by no 
means my intention to read this list, or any portions thereof, but 
as the cause of the simultaneous occurrence in different regions is 
not the same for all species, I propose to offer a few general re- 
marks on the various phases which this subject assumes. 
The simultaneous occurrence of identical species in regions 
separated by wide stretches of ocean, or other great natural 
boundaries, can only be explained, ist, by NATURAL DISPERSION ; 
* Since this was written a very carefully elaborated list of the Coleoptera 
common to North America and Europe has been published by Dr. John 
Hamilton in Trans. Amer. Ent. Soc., v. 16, 1889, pp. 88-162, which has 
been reprinted, with additions and corrections, by an equally competent 
European Entomologist, Mr. Albert Fauvel, in Revue d'Entomologie, v. 
8, 1889, pp. 92-174. 
f Fauvel enumerates 495 species. 
