Marsh—North American Species of Diaptomus. 385 
Effect of the glacial period on distribution. 
Before the glacial period, we may suppose that the waters 
of North America were peopled with species of Diaptomus. 
Inasmuch as the continental conditions differed less than those 
of the present time, it is probable that the number of species 
was smaller. This is probable, too, from the fact that the 
means of communication from one body of water to another 
was probably easy, so that there was less opportunity for the 
effect of isolation. As the period progressed, the genus must 
have been driven further and further towards the south, 
being limited, on the north, during the period, very nearly 
to the southern limit of the ice. This limit would not have 
been exact, for species of Diaptomus live in the coldest waters. 
During the decline of the glacial period it is fair to assume 
that, as the ice retreated, the Diaptomi followed towards the 
north. Inasmuch as the species of Diaptomi at the present 
time are limited in their distribution by climatic influences, we 
may assume a gradual change in the species in any given local¬ 
ity as the climate changed. The forms which had been living 
close to the ice border would retreat towards the Arctic, or 
might remain as a fauna relicta if local conditions were such 
as to make this possible. 
If the above assumptions in regard to the origin of the genus 
and its geological history are accepted, we must suppose:— 
First, that inasmuch as there were several, perhaps many, 
species before the glacial period, our modern species will prob¬ 
ably be divided into groups, according to their phytogeny, but 
it is unlikely that we can trace to any one line for all species. 
Second, the most primitive species will be the furthest north. 
We may expect to find in Arctic or sub-Arctic regions the 
forms most nearly related to those that dwelt in the United 
States during the glacial period, and these forms will be found 
further south only as a part of a fauna relicta unless they have 
been able to adapt themselves to great variations of climate; 
2o—S'. & A. 
