ENTOMOSTRACA AND LIFE ZONES. 97 



III. THE FAUNA OF EACH ZONE. 



Table II., summarizing the fauna of the different zones, shows 

 the following points of interest. For the species collected in 

 Colorado there is a decrease in number from the Upper Sonoran 

 upward, that is, a thinning out of population as the more extreme 

 conditions of climate are met. An apparent exception is the 

 Transition Zone where the number is less than in the Canadian. 

 This is no doubt due to the fact that in the region assigned to this 

 zone there are very few lakes and the number of collections is 

 accordingly much less than from any other zone. If we add to 

 the number of species actually collected from each zone in Colo- 

 rado those which from their general distribution are almost cer- 

 tainly to be expected there, the deficiency of the Transition Zone 

 is made up and the general decrease with each successive zone 

 prevails. 



TABLE II. 



SUMMARY OF RECORDS GIVEN IN TABLE I. 



The Upper Sonoran Zone. 



In the Upper Sonoran Zone of this region there occur two 

 kinds of ponds or small lakes : transient pools which follow rains, 

 and ponds and reservoirs filled with water from irrigating ditches. 

 These latter are of recent origin, dating back only to the intro- 

 duction of agriculture into this region. The fact that they are 

 richly populated speaks well for the effectiveness of the methods 

 of dispersal of plankton Crustacea, but the question may well be 

 raised whether dispersal has kept pace with the increase in num- 

 ber of such bodies of water and whether additions to their fauna 

 may not be expected in future years. 



Of the 44 species that have been recorded from this zone in 

 Colorado, 28 have not been found in any of the higher zones, 



