28 PROCEEDINGS OF THE NATIONAL. MUSEUM vol.86 



The reason for such differences as exist between the number of 

 millipeds of the two sides of the country may be understood from an 

 examination of the environmental requirements of the animals. The 

 limitations that confine groups of creatures to certain sets of con- 

 ditions vary enormously; some have limitations so lightly drawn 

 that they may inhabit great areas, while others may be restrained 

 from spreading by more rigid requirements. The limitations im- 

 posed on the millipe<:ls, and similar humus inhabitants, are especially 

 restrictive, for their movements generally are held to localities hav- 

 ing very uniform and constant supplies of food and moisture, or to 

 areas in which they may move about following or keeping within 

 such conditions. A few of the larger millipeds have been able to 

 adapt themselves to semiarid or even arid regions, as the heavily 

 armored, protective covering of their bodies allows greater freedom 

 of movement in the open, above ground, but even these species re- 

 quire some natliral protection, and where this is not provided by a 

 humus layer they retreat into deep crevices in the soil or rocks, or 

 into the burrows of other animals, and there spend much of their 

 time. By far the largest number of species are definitely humus 

 inhabitants, delicate creatures, most of them unable to withstand a 

 few moments of hot sunshine or somewhat longer exposure to ex- 

 treme dryness, whether above or below ground. 



Throughout the Eastern States, with their more general rainfall, 

 lower elevation, and abundant deciduous forests, humus conditions 

 occur frequently and over considerable areas, so that intennigration 

 and wide distribution of the humus fauna are possible. In the Western 

 States the rainfall is less uniform; many regions have long seasonal 

 droughts, others are arid deserts; the country is much more moun- 

 tainous and has higher elevations, and the forests are predominantly 

 coniferous, so that satisfactory humus conditions are generally of 

 smaller extent and more definitely separated than in the East, and 

 examples of isolation and limited distribution among the millipeds 

 are the common rule. While there are many species of restricted 

 distribution in the East, there also are many examples of widespread 

 species, such as ArctoholuH marginatum ^ Poh/Bonmm hirirgafvm, 

 Spirostrephon lactarium, and Polydesmus serratu.s, to name but a 

 few. From the literature on millipeds of the West and the col- 

 lecting that has been done there exceedingly restricted distribution 

 for most of the species is indicated, and none has been found that 

 can be compared to the widely distributed eastern species. 



Frequently a few miles separate different but closely related species 

 of western millipeds, and the evidence points to the fauna being 

 chiefly residual, descended in a large number of more or less iso- 



