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PROCEEDINGS OF THE NATIONAL MUSEUM. 



VOL, 54. 



In strong contrast is the third group, including 24 species which 

 seem about equally. at home in either mountains or plains. Of these 

 there are 22 common to both old and new worlds, and only two have 

 a range restricted to the western United States. 



Table 3 summarizes the facts presented in the preceding para- 

 graphs. 



Table 3. — Distribution of species by groups. 



Number species iu each group 



Confined to western United States 



Confined to North America 



"W orld-wide 



In the above comparisons between the euthermic and steno- 

 thermic groups of species there have doubtless been some mistakes 

 made in assigning certain species to a given group, but, even allowing 

 for some error from this source, it is quite evident that the steno- 

 thermic forms found in tliis region have a much less extended range 

 than the euthermic. Of course, inasmuch as stenothermic species 

 are of necessity shut out from large areas by unsuitable tempera- 

 tures, it is not to be expected that such species should have as wide 

 ranges as do those forms not so limited, but such differences as those 

 just pointed out can ' hardly find a complete explanation in this 

 set of conditions. 



In this connection it is of interest to compare the genera Cyclops 

 and Diaptornus, each of which is world-wide, forming an important 

 part of the fresh-water plankton Crustacea everywhere. Cyclops is 

 characterized by having a relatively small number of species, most 

 of which are euthermic in nature and have a wide geographical 

 range, while Diaptomus includes a multipHcity of species, usually 

 stenothermic and with very hmited ranges. Of the five species of 

 Cyclops found in Colorado, four are very common at aU elevations 

 and are also practically world-wide in distribution; while of the 13 

 species of Diaptomus about half belong strictly to the mountains, 

 the other haK to the plains, and not one of them has a range extend- 

 ing beyond North America, most of them being confined to a narrow 

 area in western United States. Moreover in the entire genus there 

 is not laiown a single species common to Eastern and Western 

 Hemispheres, The apparent correlation between stenothermic 

 habit and restricted range is striking, but to what extent it is of 

 general application and whether there is any necessary relation 

 between the two conditions must at present be left unanswered. 



Zonation. — If we can recognize among plants or animals ranging 

 through different chmatic conditions a zonation, we have an instruc- 

 tive method of analysis of use in bringing out significant points in 



