750 FRESH-WATER BIOLOGY 



the zoologist. Many of the species of the Cyclopidae are almost 

 if not quite cosmopolitan in their distribution; for example, Cyclops 

 leuckarti not only occurs all over North America, but in Europe, 

 Asia, and Africa, and without any variations that are character- 

 istic of the different regions. It seems very remarkable that an 

 animal as delicately organized as Cyclops should not show the 

 effect of change of location in its structure. Most of the recog- 

 nized American species of Cyclops are identical with those found 

 in other continents; and it is even possible that, when the genus 

 is known more thoroughly than at present, many of the species 

 which are now considered peculiar to our continent may be found 

 to be either identical with foreign species or at most only varia- 

 tions of those forms. So our common species of Limnocalanus, 

 L. macrurus, is identical with the European form. On the other 

 hand, not only the species but the genera of Osphranticum and 

 Epischura are peculiar to North America. The genera of the 

 Harpacticidae have never been thoroughly worked over, and, while 

 some of our species are undoubtedly European, it seems probable, 

 from what we now know, that many of them are peculiar to this 

 continent. Eurytemora is world-wide in its distribution. 



Of the Diaptomi there are now recognized thirty-nine species 

 in North America, and all of these are peculiar to this continent. 

 Not only are they peculiar to the continent, but many are peculiar 

 to certain regions. In a broad way, they illustrate very forcibly 

 what has been said before, that Copepoda are controlled in their 

 distribution by temperature conditions. This can well be illus- 

 trated by a brief discussion of the geographical distribution of the 

 more common species. Diaptomus minutus is found from Green- 

 land and Iceland south to the northern tier of states in the United 

 States, but does not occur south of 42 to 43 N. L. Diaptomus 

 sicilis is confined to the northern tier of states. Diaptomus sici- 

 loides is found in a band farther to the south, being limited roughly 

 to the region between the thirty-sixth and forty-third parallels. 

 These three species are closely related to each other in structure, 

 and presumably are of the same line of descent. It will be seen 

 that their distribution, taken in a broad way, is one of latitude. 



A similar relation exists between Diaptomus oregonensis, Diapto- 



