Geographical Range of Species. 
289 
dion. There probably are exceptions to such a rule, but it is 
still far too early and our knowledge of the group is far too de¬ 
fective to warrant us in naming this or that genus as confined 
do one continent. 
One further fact mentioned by me in an earlier paper has not 
heen sufficiently considered in connection with the geographical 
range of the species of Cladocera. This is the wide geo¬ 
graphical area over which the species may be found, coupled 
wdth a very irregular distribution in that area. As a good 
•example of this fact may be mentioned Ancliistropus minor , 
described in the second part of this paper. A single specimen 
of this species was found by me in lake Wingra in 1890. No 
second specimen could be discovered although several days were 
devoted to the search; nor has another individual been seen 
from this locality in the course of the three years which have 
passed since the first one was seen. Another single individual 
was found in Lake Winnebago in 1892 and again I was unable 
to find another even after most careful dredging in the same 
place where the first was found. Finally I found the species 
fairly abundant at Isle Royale. It can not be doubted that 
Ancliistropus minor occurs widely throughout the state of Wis¬ 
consin and yet it is so rare or local in its distribution that it 
seems a mere matter of accident whether or not it is reported 
from a given locality. This case is paralleled by many others 
in the experience of every collector of Cladocera and illustrates 
the need of extreme caution in declaring a species absent from 
a region or from any given lake. My own belief is that most 
species whose form is not very variable in the locality where 
they are found, will have an intercontinental distribution. Ex¬ 
ceptions will no doubt occur, but the presumption so far as the 
facts are known to us, is in favor of a wide area of distribution 
rather than a smaller one. 
