Geographical Range of Species. 289 



Hon. 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 

 to one continent. 



One further fact mentioned by me in an earlier paper has not 

 been 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 

 with a very irregular distribution in that area. As a good 

 example of this fact may be mentioned Anchistropus 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 dredgiug 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 

 Anchistropus 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. 



