Marsh — North American Speci-es of Diaptomus. 385 



Effect of the glacial period on clistribution. 



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 phylogeny, but 

 it is unlikely that we can trace to any one line for all specie*. 



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, 

 25— S. & A. 



