72 PROCEEDINGS OF THE NATIONAL MUSEUM. 



VOL. 54. 



drawing conclusions. Figure 10 includes the total list for the State, 

 with a graphic expression of the known range of each. Tlie sohd 

 black part of the hne represents the range as it appears from my col- 

 lections and the open part the extension of range from other Colorado 

 records. The broken lines indicate probable extensions of range into 

 elevations not covered by the present records and are based upon the 

 facts of general distribution of each species. The downward exten- 

 sion of many species below the Colorado records means that these are 

 common lowland forms in temperate latitudes and do not in Colorado 

 find their lower limit, inasmuch as even the lowest portions of the 

 State have considerable elevation. 



^ It is at once evident from the above chart that the species in this 

 list fall into three groups— (1) those confined to the plains; (2) those 

 limited to the mountains; (3) those that are not so restricted, but are 

 found at all elevations. The first two of these groups include the 

 stenothermic species, those unable to live except within rather nar- 

 row extremes of temperature. The two stenothermic groups differ 

 from each other in that while one of them is unable to withstand high 

 temperatures, the other can not tolerate low. The third group in- 

 cludes the euthermic species— those not so limited by temperature 

 conditions, but able to live about equally well within wide limits, such 

 as those between mountains and plains or between arctic conditions 

 and tropical. In comparing vertical and latitudinal distribution, the 

 first of these groups represents the arctic, or far northern, fauna; the 

 second the more southern forms; while the third is typical of the 

 species which have a wide north and south range. A brief analysis 

 win show to what extent this parallel holds and will also point out 

 that the stenothermic groups are in various respects more narrowly 

 limited than the euthermic. 



In the group confined to the mountains there are 16 species, and 

 3 others, which, though they do extend to the plains, belong pri- 

 mardy in the higher area, making 19 in all. These 19 species faU 

 into two groups: (1) Ten species vath a wide range in arctic and 

 subarctic regions (aU but one in both old and new worlds), which 

 here range southward along the higher parts of the Rocky Moun- 

 tains, a true southward extension of a northern fauna; and (2) nine 

 species (some with very narrow ranges), pretty strictly confined to 

 the Rocky Mountain region of the United States, a purely mountain 

 fauna having the characteristics of an arctic fauna but including 

 different species. 



The group confined to the plains has som.ewhat similar components. 

 Of the 28 species assigned to this group (the position of 3 is some- 

 what doubtful) 5 are found also in the old world and 6 others have 

 quite a wide range in the United States. The remaining 17 species 

 have a rather restricted range on the plains of the western part of 

 the United States, some few extending into Mexico and southern 



