400 Wisconsin Academy of Sciences, Arts, and Letters. 



not appear likely, however, that the others are derivatives of 

 this, but rather that they go back to a common ancestry which 

 resembled tenuicaudatns more nearly than the others. 



FACTORS CONTROLLING THE DISTRIBUTION OF THE DIAPTOMI. 



I think it will appear evident from the discussion of the 

 groups of the Diaptomi that there are two great factors con- 

 trolling their distribution. One is ease of water communica- 

 tion, the other is temperature. There is no reason to think 

 that, under ordinary circumstances, species are distributed in 

 any way except by water carriage. The existence of species 

 isolated in particular localities seems pretty conclusive evi- 

 dence of this ; this is seen in the peculiarly localized habitat of 

 D.. Beighardi, in the distribution of D. Birgei, and in the pres- 

 ence of D. siciloides in Cedar lake, Wisconsin, although it 

 has been found in no other part of the state. 



An examination of the distribution of the members of the 

 different groups shows very clearly the prominence of the two 

 factors mentioned above. In the oregonensis group, D. Reigh- 

 ardi, D. Bakeri and D. franciscanus are localized species. D. 

 oregonensis is a distinctly northern species, while D. missis- 

 sippiensis is as distinctly southern. D. pallidus has a some- 

 what wider range, but apparently through communicating 

 waters. 



Of the tenuicaudatus group, none are found in the South. 

 D. tenuicaudatus has been found only in the far North. D. 

 minutus has the widest range, being found from Iceland to 

 southern Wisconsin. D. sicilis and D. Ashlandi are confined 

 to the northern tier of states, while D. siciloides and D. Birgei 

 are prob&;bly limited, speaking in a very general way, to the 

 region between the parallels of 39° and 43°. The distribution 

 with reference to latitude is very marked in the case of most 

 of the members of this group. 



Of the leptopus group, D. lepiopus is the most widely dis- 

 tributed, being found not only through the Mississippi valley 

 Dut north into British America. D. clavipes and D. Lintond 

 are probably somewhat localized. D. stagnalis is found from 



