COPEPODA 751 



THUS mississippiensis, and Diaptomus pallidus. Diaptomus orego- 

 nensis is the more northern species. It is found from one side of 

 the continent to the other, as far north as the Saskatchewan region, 

 and as far south as Illinois and Indiana. Diaptomus pallidus is a 

 Mississippi Valley species, having been found from Minnesota to 

 Louisiana, and as far west as Colorado. Diaptomus mississippiensis 

 is a strictly southern species, being confined, so far as known, to 

 the Gulf States. It is evident that this distribution again is con- 

 trolled by temperature conditions. 



The group which centers around Diaptomus albuquerquensis is 

 confined to the south, the most southern species being Hmited to 

 the island of Cuba. The group centering about Diaptomus signi- 

 Cauda is confined to the mountain regions of the West, where a 

 number of rather closely related species have been developed. 

 Probably the greatest number of species is found in this mountain 

 region of the West, where the lakes are separated from each other, 

 and isolation has led to the development of new species. It will 

 thus be seen that the one great controlling factor in the distribu- 

 tion of the Diaptomi is temperature. 



It may perhaps be assumed that most of our North American 

 species are descended from the same ancestors as those of the 

 other continents; that as the result of the glacial period the north- 

 ern forms were forced far to the south; and that, on the retreat of 

 the ice, some followed after the ice, while others remained behind, 

 but changed their form as the result of the changed environment. 

 Thus the more primitive forms would be found to the north. In 

 the south we would find specialized forms due to the various fac- 

 tors which have come into play in the evolution of these animals. 

 It is a peculiar fact that in this change of conditions the Cyclo- 

 pidae should have succeeded in adapting themselves without change 

 of structure, while the Diaptomi all suffered changes. The differ- 

 ence in the behavior of these two families is a matter that is not 

 at all understood, but it seems possible that the Cyclopidae have 

 more efficient means of distribution, so that the development of 

 new species from isolation would not be as probable as in the case 

 of the Diaptomi. As a matter of fact, very Httle is known of the 

 life histories of these animals. 



