716 REPORT OF COMMISSIONER OF FISH AND FISHERIES. [16] 



est doubt that they have had a relatively recent origin in common with 

 these Old World species. On the other hand, the points of difference 

 between our species and their European representatives, though slight 

 and scarcely entitled to specific rank unless on the ground of their con- 

 stancy, indicate a separation long enough ago to permit at least incip- 

 ient differentiation. These facts seem to point to an origin connected 

 with the "glacial period" — whether immediately subsequent to that 

 period, as suggested by Herrick,* or just previous to the time of actual 

 glaciation, when the milder climate and the greater land elevation 

 northward t permitted a freer passage than now of fresh- water forms 

 across the north Atlantic, it would seem impossible to say until we 

 know more of the present northward limit of distribution of the species 

 concerned. If they now range far up into the arctic regions, it would 

 seem possible that they may have lived everywhere in the icy waters of 

 the time of diminishing glaciation ; but if their habitat is strictly 

 subarctic or temperate, their area can not have been continuous with 

 that of the European species since pre-glacial times. 



That Epischura mast have had a different history is a fact already 

 noticed, and considering the fact that its nearest known relative, 

 Heterocope, is both fresh water and marine, it is not unlikely that it 

 came to us from the sea. 



The four or five Cyclopidne of this paper, it will be noticed, are all 

 American but one ; but this large and difficult family has been far too 

 little studied to permit generalization, the current descriptions of even 

 the more abundant species not commonly being given in sufficient detail 

 to permit careful comparison. 



The list of Cladocera, on the other hand, is remarkable for the number 

 of unaltered European species which it contains, all but four of the six- 

 teen here reported being quite indistinguishable from those described 

 from Europe, while the four excepted have very closely allied Old World 



kindred. 



Finally I would remark upon the minuteness and physiological in- 

 significance of the changes which so far seem to separate several of our 

 Entomostracan species from their European representatives. If more 

 extended collections and exhaustive study should show that the dif. 



* List of the Fresh- Water and Marino Crustacea of Alabama, with Descriptions of 

 the New Species and Synoptical Keys for identification, p. 49. 



t " The progress of events seems to have been about as follows : In the warm period 

 preceding the Glacial epoch, when the vegetation of the temperate zone flourished 

 about the north pole, there was land connection between the continents, permitting 

 the larger species of the Old World to migrate to North America. At the same time 

 the conditions in North America were favorable to the tropical species of animals 

 which had developed and flourished in South America. The refrigeration of the cli- 

 mate on the approach of tbe Glacial period, and the advance of the ice from the north, 

 cut off retreat to the Old World species, and gradually hemmed them in over the 

 southern portion of the continent, where all forms of life were compelled to re-adjnst 

 themselves to -iiew conditions. — (C. Frederick Wright, in "The Ice Age in North 

 America," p. 387.) 



