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 tenuicaudatus 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, Reiglnardi, 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. Reigk- 
ardi, D. Balceri 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 probably 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. leptopus is the most widely dis¬ 
tributed, being found not only through the Mississippi valley 
out north into British America. D. clavipes and D. Lintoni 
are probably somewhat localized. D. stagnalis is found from 
