ENTOMOSTRACA AND LIFE ZONES. IO5 



and isolated fragments of the broader life zones, from Transition 

 to Arctic-Alpine, which extend from east to west across the con- 

 tinent. Though these zones have been established and denned 

 on the basis of organisms other than entomostraca, yet the agree- 

 ment is striking and real and extends even to common species 

 inhabiting the same zones in localities hundreds of miles apart 

 and separated by many degrees of latitude. 



In this fact we are brought again to a recognition of the truth 

 that change in elevation may, within a very few miles, produce 

 climatic and biotic differences which are brought about by lati- 

 tude, only after hundreds of miles. It shows clearly that life 

 zones are neither latitudinal, nor altitudinal, though these are the 

 two large factors which interact to produce them. We have not 

 discovered an exact mathematical formula for determining life 

 zones on the basis of these two factors, nor are we able to ac- 

 curately define them on the basis of climatic conditions, but they 

 may be defined and compared in terms of distribution of ani- 

 mals and plants. These, of course, serve as a measure of climatic 

 conditions, that is, as these conditions determine the distribution 

 of the several species. It is apparently not the case that tem- 

 perature acts in exactly the same way on all species of animals 

 and plants in determining their distribution. For some, it is ap- 

 parently winter conditions which limit their range into colder 

 regions, though for the great mass, it is probably summer condi- 

 tions. It is not the purpose of this paper to enter into a discus- 

 sion of this point, though the fact of winter-killing of certain 

 species is well known, while it is recognized also that every species 

 requires a certain minimum heat budget during the summer, ex- 

 tending over a longer or shorter period, in order that it may 

 reproduce. 



It is not to be expected that limiting lines for all species will 

 be exactly parallel, yet there is a large agreement, so that the 

 zones do, in a high degree, maintain their several identities in 

 whatever proportions altitude and latitude interact in producing 

 them. It would be of interest to investigate further into the ex- 

 tent and nature of this identity between the colder zones of the 

 elevated parts of Colorado and the less elevated portions of these 

 zones in the far north. The present study goes merely far enough 



